Novel protein pores

ABSTRACT

The present invention relates to novel protein pores and their uses in analyte detection and characterisation. The invention particularly relates to an isolated pore complex formed by a CsgG-like pore and a modified CsgF peptide, or a homologue or mutant thereof, thereby incorporating an additional channel constriction or reader head in the nanopore. The invention further relates to a transmembrane pore complex and methods for production of the pore complex and for use in molecular sensing and nucleic acid sequencing applications.

RELATED APPLICATIONS

This application is a Continuation of U.S. application Ser. No. 16/624,341, filed Dec. 19, 2019, entitled “NOVEL PROTEIN PORES”, which is a national stage filing under 35 U.S.C. 371 of International Patent Application Serial No. PCT/GB2018/051858, filed Jul. 2, 2018, entitled “NOVEL PROTEIN PORES”. Foreign priority benefits are claimed under 35 U.S.C. § 119(a)-(d) or 35 U.S.C. § 365(b) of European application number 17179099.1, filed Jun. 30, 2017. The entire contents of these applications are incorporated herein by reference in their entirety.

FIELD

The present invention relates to novel protein pores and their uses in analyte detection and characterisation. The invention further relates to a transmembrane pore complex and methods for production of the pore complex and for use in molecular sensing and nucleic acid sequencing applications.

BACKGROUND

Nanopore sensing is an approach to analyte detection and characterization that relies on the observation of individual binding or interaction events between the analyte molecules and an ion conducting channel. Nanopore sensors can be created by placing a single pore of nanometer dimensions in an electrically insulating membrane and measuring voltage-driven ion currents through the pore in the presence of analyte molecules. The presence of an analyte inside or near the nanopore will alter the ionic flow through the pore, resulting in altered ionic or electric currents being measured over the channel. The identity of an analyte is revealed through its distinctive current signature, notably the duration and extent of current blocks and the variance of current levels during its interaction time with the pore. Analytes can be organic and inorganic small molecules as well as various biological or synthetic macromolecules and polymers including polynucleotides, polypeptides and polysaccharides. Nanopore sensing can reveal the identity and perform single molecule counting of the sensed analytes, but can also provide information on the analyte composition such as nucleotide, amino acid or glycan sequence, as well as the presence of base, amino acid or glycan modifications such as methylation and acylation, phosphorylation, hydroxylation, oxidation, reduction, glycosylation, decarboxylation, deamination and more. Nanopore sensing has the potential to allow rapid and cheap polynucleotide sequencing, providing single molecule sequence reads of polynucleotides of tens to tens of thousands bases length.

Two of the essential components of polymer characterization using nanopore sensing are (1) the control of polymer movement through the pore and (2) the discrimination of the composing building blocks as the polymer is moved through the pore. During nanopore sensing, the narrowest part of the pore forms the reader head, the most discriminating part of the nanopore with respect to the current signatures as a function of the passing analyte. CsgG was identified as an ungated, non-selective protein secretion channel from Escherichia coli (Goyal et al., 2014) and has been used as a nanopore for detecting and characterising analytes. Mutations to the wild-type CsgG pore that improve the properties of the pore in this context have also been disclosed (WO2016/034591, WO2017/149316, WO2017/149317 and WO2017/149318, PCT/GB2018/051191, all incorporated by reference herein).

For analytes being polynucleotides, nucleotide discrimination is achieved via passage through such a mutant pore, but current signatures have been shown to be sequence dependent, and multiple nucleotides contributed to the observed current, so that the height of the channel constriction and extent of the interaction surface with the analyte affect the relationship between observed current and polynucleotide sequence. While the current range for nucleotide discrimination has been improved through mutation of the CsgG pore, a sequencing system would have higher performance if the current differences between nucleotides could be improved further. Accordingly, there is a need to identify novel ways to improve nanopore sensing features.

SUMMARY

The disclosure relates to modified CsgF peptides, in particular truncated CsgF fragments, binding the CsgG pore and thereby introducing another additional channel or pore constriction within the CsgG pore. Other aspects of the invention also relate to an isolated transmembrane pore complex, and the use of said CsgG:CsgF complexes and modified CsgF peptides or fragments in a nanopore sensing platform with two consecutive reader heads.

The first aspect of the invention relates to a pore comprising a CsgG pore and a CsgF peptide. In one aspect, the CsgF peptide comprises a CsgG-binding region, and a region that forms a constriction in the pore. In one aspect the CsgF peptide is a truncated CsgF peptide lacking the C-terminal head domain of CsgF. In another aspect the CsgF peptide is a truncated CsgF peptide lacking the C-terminal head and a portion of the neck domain of CsgF. In another aspect the CsgF peptide is a truncated CsgF peptide lacking the C-terminal head and neck domains of CsgF. The pore is also referred to herein as a pore complex and as an isolated pore complex. The isolated pore complex comprising a CsgG pore, or a homologue or mutant thereof, and a modified CsgF peptide, or a homologue or mutant thereof, in particular truncated CsgF fragments, or homologues or mutants thereof. In one embodiment, said modified CsgF peptide, or homologues or mutants, is located in the lumen of the CsgG pore, or homologues or mutants thereof. In another embodiment, said isolated pore complex has two or more channel constrictions, one located or provided by the CsgG pore, formed by its constriction loop, and another additional channel constriction or reader head, introduced by the modified CsgF peptide or its homologues or mutants. In one embodiment, said CsgG-pore or CsgG-like pore, is not a wild-type pore, it is a mutant CsgG pore, with in particular embodiments mutations being present, for example, in said channel constriction loop. In another embodiment, the isolated pore complex, comprising the modified CsgF peptide, or a homologue or mutant thereof, has a CsgF channel constriction with a diameter in the range from 0.5 nm to 2.0 nm. In one embodiment, the pore complex comprises: (i) a CsgG pore comprising a first opening, a mid-section comprising a beta barrel, a second opening, and a lumen extending from the first opening through the mid-section to the second opening, wherein a luminal surface of the mid-section defines a CsgG constriction; and (ii) a plurality of modified CsgF peptides, each having a CsgF constriction region and a CsgF binding region (also referred to herein as a CsgG-binding domain or region of CsgF), wherein the modified CsgF peptides form a CsgF constriction within the beta barrel of the CsgG pore and wherein the CsgG constriction and the CsgF constriction are co-axially spaced apart within the beta barrel of the CsgG pore. The luminal surface of the CsgG pore may comprise one or more loop regions of CsgG monomers that define the CsgG constriction. The CsgF constriction region and the CsgF binding region typically correspond to a N-terminal portion of a CsgF mature peptide. In one embodiment, the pore complex excludes CsgA, CsgB and CsgE.

In a second aspect, the invention relates to a modified CsgF peptide, or a homologue or mutant thereof, wherein the protein or peptide is modified via a truncation or deletion of part of the protein, resulting in a CsgF fragment of SEQ ID NO:6 or of a homologue or mutant thereof. One embodiment relates to a modified or truncated CsgF peptide, or a modified peptide of a CsgF homologue or mutant, said modified peptide comprising SEQ ID NO:39, or SEQ ID NO:40, or a homologue or mutant thereof, alternatively, said modified peptide comprises SEQ ID NO:15, or a homologue or mutant thereof, alternatively SEQ ID NO: 54, or SEQ ID NO: 55 or a homologue or mutant thereof. Another embodiment discloses a modified CsgF peptide wherein one or more positions in the region comprising SEQ ID NO:15 are modified, and wherein said mutation(s) required to retain a minimal of 35% amino acid identity to SEQ ID NO:15, in the peptide fragment corresponding to the region comprising SEQ ID NO:15.

One embodiment relates to a pore comprising a CsgG pore and a modified CsgF peptide, wherein the modified CsgF peptide is bound to CsgG and forms a constriction in the pore

One embodiment relates to a polynucleotide encoding said modified CsgF peptide, or homologue or mutant thereof, according to the second aspect of the invention. In another embodiment, the isolated pore complex comprising a CsgG pore and a modified CsgF peptide, or homologues or mutants thereof, are characterized in that said modified CsgF peptide is a peptide provided by the peptides disclosed in the second aspect of the invention.

Another embodiment relates to the isolated pore complex wherein the modified CsgF peptide and the CsgG pore or a monomer of said pore, or homologues or mutants thereof, are covalently coupled. And even more particularly, said coupling is made via a cysteine residue or via a non-native reactive or photo-reactive amino acid in a CsgG monomer at a position corresponding to 132, 133, 136, 138, 140, 142, 144, 145, 147, 149, 151, 153, 155, 183, 185, 187, 189, 191, 201, 203, 205, 207 or 209 of SEQ ID NO: 3, or of a homologue thereof.

A preferred embodiment relates to an isolated transmembrane pore complex, or a membranous composition, which comprises the isolated pore complex of the invention, and the components of a membrane. Particularly, said transmembrane pore complex or membranous composition consists of the isolated pore complex of the invention, and the components of a membrane or an insulating layer.

One embodiment relates to a method for producing a pore as disclosed herein, the method comprising co-expressing one or more CsgG monomers as disclosed herein and a CsgF peptide as disclosed herein in a host cell, thereby allowing transmembrane pore complex formation in the cell. The CsgF peptide may be produced in the cell by cleaving a modified CsgF peptide or protein comprising an enzyme cleavage site at a suitable position in the amino acid sequence.

One embodiment relates to a method for producing a pore as disclosed herein, the method comprising contacting one or more purified CsgG monomers with one or more purified modified CsgF peptides, thereby allowing in vitro formation of the pore. The modified CsgF peptide may be a peptide comprising an enzyme cleavage site at a suitable position in the amino acid sequence, that is cleaved before or after formation of the pore.

A third aspect of the invention relates to a method for producing said transmembrane pore complex, wherein the pore is an isolated complex formed by a CsgG pore, or a homologue or mutant thereof, and a modified CsgF peptide, or a homologue or mutant thereof, the method comprising the steps of co-expressing CsgG SEQ ID NO:2, or a homologue or mutant thereof, and modified or truncated CsgF, comprising a fragment of SEQ ID NO:5, or a homologue or mutant thereof, in a suitable host cell, thereby allowing in vivo pore complex formation. In specific embodiments, said modified CsgF peptide, or homologue or mutant thereof, comprises SEQ ID NO: 12 or SEQ ID NO:14, or a homologue or mutant thereof. Alternatively, a method for producing an isolated pore complex comprises the steps of contacting CsgG monomers of SEQ ID NO:3, or a homologue or mutant thereof, with modified CsgF peptide(s), or a homologue or mutant thereof, for in vitro reconstitution of the pore complex. In particular embodiments, modified CsgF peptides of said method comprise SEQ ID NO:15 or SEQ ID NO:16, or homologues or mutants thereof.

Another aspect of the invention relates to a method for determining the presence, absence or one or more characteristics of a target analyte, comprising the steps of:

(i) contacting the target analyte with said isolated pore complex or transmembrane pore complex, such that the target analyte moves into the pore channel; and

(ii) taking one or more measurements as the analyte moves through the pore channel and thereby determining the presence, absence or one or more characteristics of the analyte.

In one embodiment, said analyte is a polynucleotide. In particular, said method using a polynucleotide as an analyte alternatively comprises determining one or more characteristics selected from (i) the length of the polynucleotide, (ii) the identity of the polynucleotide, (iii) the sequence of the polynucleotide, (iv) the secondary structure of the polynucleotide and (v) whether or not the polynucleotide is modified.

In another embodiment, the analyte is a protein, or peptide, and in further embodiments, said analyte is a polysaccharide, or a small organic or inorganic compound, such as for instance but not limited to pharmacologically active compounds, toxic compounds and pollutants.

In another embodiment, a method for characterising a polynucleotide or a (poly)peptide using an isolated transmembrane pore complex is described, wherein the pore complex is an isolated complex comprising a CsgG pore, or a homologue or mutant thereof, and a modified CsgF peptide, or a homologue or mutant thereof. In particular, said CsgG pore, or homologue or mutant thereof, comprising six to ten CsgG monomers forming the CsgG pore channel.

A further aspect of the invention discloses the use of said isolated pore complex or transmembrane pore complex according to the previous aspects of the invention to determine the presence, absence or one or more characteristics of a target analyte. Furthermore, the invention also relates to a kit for characterising a target analyte comprising (a) said isolated pore complex and (b) the components of a membrane.

DESCRIPTION OF THE FIGURES

The drawings described are only schematic and are non-limiting. In the drawings, the size of some of the elements may be exaggerated and not drawn on scale for illustrative purposes.

FIGS. 1A-1E. Structure of CsgG pore and the interface for complex formation with CsgF. Cross-sectional (FIG. 1A), side (FIG. 1B) and top (FIG. 1C) views of CsgG oligomers (e.g., nonamers) (gold) in surface (FIG. 1A) and ribbon (FIG. 1B, FIG. 1C) representation, with a single CsgG protomer colored light blue (FIG. 1D) (based on the CsgG X-ray structure PDB entry: 4uv3). The CsgG constriction loop (CL loop) spans residues 46 to 61 according SEQ ID NO:3, and is indicated in dark grey in all panels, and corresponds to the loop provided in the bottom left of FIG. 1E. CsgG residues for which the side chain faces the inner lumen of the CsgG beta-barrel are colored mid-grey as indicated and labelled in the p strands in FIG. 1E and FIG. 1D. These residues represent sites that can be used for substitution to natural or non-natural amino acids, e.g., amenable for attachment (e.g., covalent crosslinking) of a pore-resident peptide, (including e.g., a modified CsgF peptide, or a homologue thereof) to a CsgG pore or monomer. In some embodiments crosslinking residues include Cys and reactive and photo-reactive amino acids, acids such as azidohomoalanine, homopropargylglycyine, homoallelglycine, p-acetyl-Phe, p-azido-Phe, p-propargyloxy-Phe and p-benzoyl-Phe (Wang et al. 2012; Chin et al. 2002) and can be substituted into positions 132, 133, 136, 138, 140, 142, 144, 145, 147, 149, 151, 153, 155, 183, 185, 187, 189, 191, 201, 203, 205, 207 or 209 according to SEQ ID NO:3. FIG. 1E shows a zoom of the CL loop and the transmembrane beta-strands of a CsgG monomer. The CsgG constriction loop (colored dark blue) forms the orifice or narrowest passage in the CsgG pore (FIG. 1A). In some embodiments, three positions in the CL loop, 56, 55 and 51 according to SEQ ID NO:3, are of particular importance to the diameter and chemical and physical properties of the CsgG channel orifice or “reader head”. These represent preferred positions to alter the nanopore sensing properties of CsgG pores and homologues.

FIGS. 2A-2B. CsgG:CsgF complex protein co-expression and complex purification. FIG. 2A. Schematic representation of the purification protocol of the CsgG:CsgF complex starting from an E. coli culture co-expressing CsgG (SEQ ID NO:2+C-terminal StrepII tag) and CsgF (SEQ ID NO:4+C-terminal 6×His tag). The protocol involves disrupting resuspended cells and performing a 1% DDM extraction of the membrane-bound proteins. The CsgG:CsgF complex and excess CsgF undergo a first enrichment by affinity purification on a nickel IMAC column, followed by a second affinity-based enrichment of the CsgG:CsgF complex on a Streptavidin column. FIG. 2B. Coommassie stained SDS-PAGE of the IMAC (left) and Streptavidin (right) purification steps. Protein bands corresponding to CsgG and CsgF are labelled. Notably, the IMAC eluate contains a N-terminally truncated CsgF fragment (labelled *) that was not retained in the affinity pulldown using the CsgG-bound Strep tag, indicating that the CsgF N-terminus is required for complex formation with CsgG.

FIGS. 3A-3C. CsgG:CsgF complex protein purification upon in vitro reconstitution. FIG. 3A. Superimposed chromatograms of Size Exclusion Chromatography (SEC) runs (using BioRad Enrich 650 10/300 column) of CsgG (light grey) and CsgG supplemented with an excess CsgF (dark grey). The chromatograms show elution peaks corresponding to a CsgG 9-mer (a) and CsgG 18-mer (b) for the CsgG run; and an excess free CsgF (c), as well as a 9-mer CsgG:CsgF complex (d) and 18-mer CsgG:CsgF complex (e) that elute at higher hydrodynamic radius (molecular mass) due the incorporation of CsgF in the complex. FIG. 3B. Native PAGE analysis of the representative species labelled in panel (A), confirming the shift to higher molecular mass due to incorporation of CsgF into the CsgG 9-mer and CsgG 18-mer complexes. These experiments demonstrate that CsgG:CsgF complex can be reconstituted in vitro starting from the purified components. FIG. 3C. Ribbon representation of the CsgG 9-mer and CsgG 18-mer as previously reported in Goyal et al. 2014 (PDB entry 4uv3). The CsgG 18-mer is formed of a dimer of CsgG 9-mers. The SEC and native PAGE analyses shown in panels A and B demonstrate that the CsgG 18-mer is amenable to complex formation with CsgF.

FIGS. 4A-4B. CsgG:CsgF structure as determined in cryo-EM. FIG. 4A. A cryo electron micrograph of the CsgG:CsgF complex shows the presence of 9-mer and 18-mer CsgG:CsgF complexes, with a number of single particles of the 9- and 18-mer forms highlighted by full and dashed circles, respectively. FIG. 4B. Two representative class averages of the CsgG:CsgF 9-mer complex, viewed from the side. Class averages include 6020 and 4159 individual particles, respectively. The class averages reveal the presence of additional density on top of the CsgG particle, corresponding to an oligomeric complex of CsgF. Three distinct regions can be seen in the CsgF oligomer: a “head” and “neck” region, as well as a region that resides inside lumen of the CsgG beta-barrel and forms a constriction or narrow passage (labelled F) that is stacked on top of the constriction formed by the CsgG CL loop (labelled G). This latter CsgF region is referred to as CsgF Constriction Peptide (FCP).

FIG. 5. Three-dimensional structural model of a CsgG:CsgF complex. Cross-sectional views of the 3D cryoEM electron density of the CsgG:CsgF 9-mer complex calculated from 20.000 particles assigned to 21 class averages. The right picture shows a superimposition with the CsgG 9-mer X-ray structure (PDB entry: 4uv3) docked into the cryoEM density. The regions corresponding to CsgG, CsgF and the CsgF head, neck and FCP domains are indicated. The cross-sections show the CsgF FCP regions forms an additional constriction (labelled F) in the CsgG channel, approximately 2 nm above the CsgG constriction loop (labelled G).

FIGS. 6A-6C. Schematic representation of the CsgG:CsgF pore complex based on the cryo-EM structure. FIG. 6A. Schematic representation of the CsgG nanopore in cross-sectional view, harbouring a single constriction, labelled (1). CsgG-based nanopores form 3.5-4 nm wide channels that contain a 0.5-1.5 nm orifice made by the CsgG constriction loop (residues 46 to 61 according SEQ ID No:3). When in complex with CsgF, a second constriction or orifice is introduced into the CsgG channel, labelled (2)/F, and the channel exit becomes occluded by the CsgF head domain (see FIG. 5). When using modified CsgF peptides, e.g., corresponding to a CsgF constriction peptide (FCP), which lacks the neck and head regions, a CsgG:CsgF pore complex is formed with two consecutive channel constrictions or orifices ((1) and (2)), as shown in the cross-sectional view of the CsgG:CsgF cryo-EM density in FIG. 6B and in schematic representation in FIG. 6C. Removal of the neck and head regions in the modified CsgF peptide alleviates their blockage of the channel exit.

FIGS. 7A-7C. Schematic representation of the use of CsgG:CsgF pore complex for nanopore sensing applications of a (bio)polymer (FIG. 7A) or single molecule analytes (FIG. 7B). When used in polymer sensing, the second channel constriction introduced by the modified CsgF peptide increases the contact region with the analyte and forms a second interaction site and reader head. When used in single molecule nanopore sensing, the second channel constriction introduced by the modified CsgF peptide, creates a second independent analyte interaction site. FIG. 7C. Schematic representation of theoretical channel conductance profiles of small molecules (indicated by hexagonals or triangles) passing and interacting with the consecutive CsgG (1) and CsgF (2) constrictions or reader heads.

FIG. 8. Multiple sequence alignments of exemplary CsgF homologues. Aligned sequences are shown as mature proteins (i.e. lacking their N-terminal signal peptide (SP)). The boxed sequences indicate a CsgF region of sequence conservation (between 35 and 100% pairwise sequence identity—see FIG. 10) that corresponds to a CsgF constriction peptide (FCP) in some embodiments. CsgF homologues included in the multiple sequence alignment are Q88H88; A0A143HJA0; Q5E245; Q084E5; FOLZU2; A0A136HQR0; A0A0W1SRL3; B0UH01; Q6NAU5; G8PUY5; A0A0S2ETP7; E311Z1; F3Z094; A0A176T7M2; D2QPP8; N2IYT1; W7QHV5; D4ZLW2; D2QT92; A0A167UJA2. The FCP regions of E. coli CsgF (SEQ ID No:15) and the shown CsgF homologues correspond to SEQ ID Nos 18-36.

FIGS. 9A-9C. Experimental evaluation of the E. coli CsgF region forming the CsgG-interaction sequence and CsgF constriction peptide (FCP). FIG. 9A shows the mature sequences (i.e. after removal of the CsgF signal peptide, corresponding to residues 1-19 of SEQ ID NO:5) of the four N-terminal CsgF fragments (SEQ ID NO:8_CsgF residues 1-27; SEQ ID NO: 10; SEQ ID NO: 12 and SEQ ID NO: 14) that were co-expressed with E. coli CsgG (SEQ ID NO:2). FIG. 9B. Anti-Strep (left) and anti-His (right) Western blot analysis of SDS-PAGE runs of crude cell lysates of CsgG and CsgF co-expression experiments. Anti-strep analysis demonstrates the expression of CsgG in all co-expression experiments, whereas anti-his western blot analysis shows detectible levels of CsgF fragments only for the truncation mutant CsgF 1-64 (SEQ ID NO: 14). A His-tagged nanobody (Nb) was used as positive control. FIG. 9C. Anti-His dot blot analysis of the presence of CsgF fragments in CsgG:CsgF co-expression experiments. Top row shows whole cell lysates, middle and bottom rows show the eluate and flowthrough of a Strep affinity pulldown experiment. These data demonstrate that CsgF fragment 1-64, and to a much lesser extent CsgF 1-48, is specifically pulled down as a complex with Strep-tagged CsgG. CsgF fragments 1-27 and 1-38 do not result in detectable levels of the corresponding CsgF fragments and show no sign of complex formation with CsgG.

FIG. 10. Multiple sequence alignment of the CsgF region forming the CsgG-interaction sequence and CsgF constriction peptide (FCP). The figure shows a multiple sequence alignment and consensus sequence of the CsgF peptides and known homologues thereof in the region that corresponds to the CsgG-interaction. CsgF homologues are defined by the PFAM domain PF03783. These peptides bind CsgG, and localize to the lumen of the CsgG β-barrel where they form an additional constriction in the CsgG channel. These peptides and homologues thereof are examples of CsgF Constriction Peptides or FCPs. The pairwise sequence identity in the shown FCPs ranges between 35 and 98%.

FIGS. 11A-11D. The high resolution cryoEM structure of the CsgG:CsgF complex. CsgG is shown in light grey and CsgF is shown in dark grey. FIG. 11A. Final electron density map of the CsgG:CsgF complex at 3.4 Å resolution. Side view. FIG. 11B. Top view of the cryoEM structure to show CsgG:CsgF comprises a 9:9 stoichiometry, with C9 symmetry. FIG. 11C. Internal architecture of the CsgG:CsgF complex. GC, CsgG constriction, FC, CsgF constriction. FIG. 11D. Interactions between CsgG and CsgF proteins. CsgG and the CsgG constriction are coloured light grey and grey respectively. CsgF is coloured dark grey. Residues in CsgG and CsgF are labelled in light grey and black respectively.

FIG. 12. Two reader heads of the CsgG:CsgF complex. CsgG is shown in light grey and reader head of the CsgG pore is shown in dark grey. CsgF is shown in black and the reader head of the CsgF is labelled.

FIG. 13. Co-expression of CsgG with CsgF WT in vivo. Gene encoding the C-terminal strep tagged CsgG polypeptide in pT7 vector with ampicillin resistance and the gene encoding the C-terminal His tagged CsgF polypeptide in pRham vector with kanamycin resistance were transformed together into E. coli BL21 DE3 cells in the presence of both ampicillin and kanamycin. Proteins were expressed for overnight at 18° C. at 250 rpm and the CsgG-CsgF complex was purified using the Strep tag purification followed by His tag purification. (Gel A) Protein sample (in duplicate) before strep purification. (Gel B) Protein sample (three elution fractions) after His purification. Proteins were run on a 4-20% Tris gel.

FIG. 14. Co-expression of CsgG and CsgF in vitro and the heat stability of the CsgG-CsgF complex. CsgG and CsgF DNA in different vectors are co-expressed in an in vitro transcription and translation reaction. Proteins are radiolabeled with S-35 methionine and exposed onto a X-ray film. Stability of the complex is assessed by incubating the reaction mixture for 10 minutes at different temperatures.

FIGS. 15A-15B. Making CsgG:CsgF complexes using protease cleavage sites. FIG. 15A. TEV or C3 or any other protease cleavage site can be Incorporated into the CsgF peptide at required sites (eg: between 30 and 31, 35 and 36, 40 and 41, 45 and 46 of seq ID no. 6) CsgG is shown in gold and CsgF domains in red. 1-35 of one CsgF subunit is coloured in green for clarity. 36-45 is shown in purple. 10 Histidine tag is shown in pink and the strep tag on CsgG is shown in blue. FIG. 15B. SDS-PAGE (4-20% TGX) for protease cleavage of the full length CsgG:CsgF complex where TEV protease cleavage site is inserted between 35-36 of seq ID 6. M:molecular weight marker, Lane 1: post strep purification of CsgG:CsgF full length complex, Lane 2: post strep concentrated, Lane 3: post gel filtrated, Lane 4: cleaved with TEV protease to generate CsgG:CsgF complex, Lane 5: flow through of CsgG:CsgF after strep purification, Lane 6: CsgG:CsgF heated at 60° C. for 10 minutes. Lane 7: Eluted CsgG:CsgF complex from strep column, Lane 8: CsgG pore as the control, Lane 9: TEV protease as the control.

FIGS. 16A-16C. Heat stability of CsgG:CsgF complexes. M: Molecular weight marker, Lane 1: CsgG pore, Lane 2: CsgG:CsgF complex at room temperature: Lanes 3-9: CsgG:CsgF sample was heated at different temperatures (40, 50, 60, 70, 80, 90, 100° C. respectively) for 10 minutes. Lane 1:

FIG. 16A. Y51A/F56Q/N55V/N91R/K94Q/R97W-del(V105-I107):CsgF-(1-45). FIG. 16B. Y51A/F56Q/N55V/N91R/K94Q/R97W-del(V105-I107):CsgF-(1-35). FIG. 16C. Y51A/F56Q/N55V/N91R/K94Q/R97W-del(V105-I107):CsgF-(1-30).

Samples were subjected to SDS-PAGE on a 7.5% TGX gel. CsgG:CsgF complexes with both CsgF-(1-45) and CsgF-(1-35) shows a shift from the CsgG pore band in lanes 1. Therefore, it is clear that both those complexes are heat stable up to 90° C. The complex and the pore breaks down to CsgG monomers at 1000 (lanes 9). Although the same heat stability pattern is seen with the CsgG:CsgF complex with CsgF-(1-30), its difficult to see the shift between the protein bands of the CsgG pore (lane 1) and CsgG-CsgF complexes (lanes 2-8).

FIG. 17. CsgG:CsgF formation via in vitro reconstitution using synthetic CsgF peptides. Native PAGE showing CsgG:CsgF formation via in vitro reconstitution using wildtype CsgG or a CsgG mutant with altered constriction Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107). An Alexa 594-labelled CsgF peptide corresponding to the first 34 residues of mature CsgF (Seq ID No 6) was added to purified Strep-tagged CsgG or Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107) in 50 mM Tris, 100 mM NaCl, 1 mM EDTA, 5 mM LDAO/C8D4 in a 2:1 molar ratio during 15 minutes at room temperature to allow reconstitution. After pull down of CsgG-strep on StrepTactin beads, the sample was analysed on native-PAGE. Both WT and Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107) CsgG bind the CsgF N-terminal peptide as visualised by the fluorescence tag.

FIGS. 18A-18B. Stabilising CsgG:CsgF or CsgG:FCP complexes. FIG. 18A. Identified amino acid positions of CsgG (SEQ ID NO: 3 and CsgF (SEQ ID NO:. 6) pairs where S—S bonds can be made. FIG. 18B. Schematic representation to show the S—S bond between CsgG-Q153C and CsgF-G1C.

FIGS. 19A-19B. Cysteine cross linking of the CsgG:CsgF complex. FIG. 19A. Y51A/F56Q/N91R/K94Q/R97W/Q153C-del(V105-I107) and CsgF-G1C proteins were purified separately and incubated together at 4° C. for 1 hour or overnight to form the complex and allow S—S formation. No oxidising agents were added to promote S—S formation. Control CsgG pore (Y51A/F56Q/N91R/K94Q/R97W/Q153C-DEL(V105-I107)) and complex (with and without DTT) were heated at 100° C. for 10 minutes to breakdown the complex into CsgG monomer (CsgG_(m), 30KDa) and CsgF monomer (CsgF_(m), 15KDa). A dimer between the CsgG_(m) and CsgF_(m) (CsgG_(m)-CsgF_(m), 45KDa) can be seen in the absence of the reducing agents confirming the S—S bond formation. Increased dimer formation can be seen in overnight incubation compared to one hour incubation. FIG. 19B. Mass spectrometry analysis was carried out on the gel purified CsgG_(m)-CsgF_(m) band from overnight incubation. Protein was proteolytically cleaved to generate tryptic peptides. LC-MS/MS sequencing methods were performed, resulting in the identification of the precursor ion above, corresponding to the linked peptides shown. This precursor ion was fragmented to give the fragment ions observed. These include ions for each of the peptides, as well as fragments incorporating the intact disulphide bond. This data provides strong evidence for the presence of a disulphide bond between C1 of CsgF and C153 of CsgG.

FIG. 20. Improving the efficiency of Cysteine cross linking of the CsgG:CsgF complex. Lane 1: Y51A/F56Q/N91R/K94Q/R97W/N133C-del(V105-I107) and CsgF-T4C proteins were co expressed the CsgG:CsgF complex was purified. Lane 2: The complex was heated in the presence of DTT to break down the complex into substituent monomers (CsgG_(m) and CsgF_(m)). DTT will break down any S—S bonds between CsgG-N133C and CsgF-T4C if formed. Lane 3: The complex is incubated with the oxidising agent copper-orthophenanthroline to promote S—S bond formation. Lane 4: Oxidised sample was heated at 100° C. in the absence of DTT to break down the complex. A new band of 45KDa corresponding to the CsgG_(m)-CsgF_(m) appears confirming the S—S bond formation.

FIGS. 21A-21B. Current signature when the DNA strand is passing through the CsgG:CsgF complex. The complexes were made by co-expressing the CsgG pore (Y51A/F56Q/N91R/K94Q/R97W-del(V105-I107)) containing the C terminal strep tag with the full length CsgF proteins containing C terminal His tag and TEV protease cleavage site between 35 and 36 of seq ID no. 6. Purified complexes were then cleaved by TEV protease to make the given CsgG:CsgF complexes. Note that TEV cleavage leaves ENLYFQ sequence at the cleavage site. FIG. 21A. No mutations at 17 position of CsgF. FIG. 21B. N17S mutation in CsgF.

FIGS. 22A-22B. Current signature when the DNA strand is passing through the CsgG:CsgF complex. The complexes were made by incubating Y51A/N55V/F56Q/N91R/K94Q/R97W-del(V105-I107) pore containing the C terminal strep tag with CsgF-(1-35) mutants. FIG. 22A. CsgF-N17S-(1-35). FIG. 22B. CsgF-N17V-(1-35).

FIGS. 23A-23F. Current signature when the DNA strand is passing through the CsgG:CsgF complex. The complexes were made by incubating different CsgG pores containing the C terminal strep tag with CsgF-N17S-(1-35). FIG. 23A. CsgG pore is Y51A/N55V/F56Q/N91R/K94Q/R97W-del(V105-I107). FIG. 23B. CsgG pore is Y51T/N55V/F56Q/N91R/K94Q/R97W-del(V105-I107). FIG. 23C. CsgG pore is Y51A/N551/F56Q/N91R/K94Q/R97W-del(V105-I107). FIG. 23D. CsgG pore is Y51A/F56A/N91R/K94Q/R97W-del(V105-I107). FIG. 23E. CsgG pore is Y51A/F561/N91R/K94Q/R97W-del(V105-I107). FIG. 23F. CsgG pore is Y51S/N55V/F56Q/N91R/K94Q/R97W-del(V105-I107).

FIGS. 24A-24C. Current signature when the DNA strand is passing through the CsgG:CsgF complex. Complexes were made by incubating the E. coli purified Y51A/N55V/F56Q/N91R/K94Q/R97W-del(V105-I107) pore containing the C terminal strep with CsgF of three different lengths. FIG. 24A. CsgF-(1-29), FIG. 24B. CsgF-(1-35), FIG. 24C. CsgF-(1-45). The arrow indicates the range of the signal. Surprisingly, complex with the CsgF-(1-29) produces the signal with the largest range.

FIG. 25. Signal:noise of the current signature when the DNA strand is passing through the CsgG:CsgF complex. Different CsgG:CsgF complexes were made by incubating different CsgG pores (1-Y51A/F56Q/N91R/K94Q/R97W-del(V105-I107) 2-Y51A/N551/F56Q/N91R/K94Q/R97W-del(V105-I107) 3-Y51A/N55V/F56Q/N91R/K94Q/R97W-del(V105-I107) 4-Y51A/F56A/N91R/K94Q/R97W-del(V105-I107) 5-Y51A/F561/N91R/K94Q/R97W-del(V105-I107) 6-Y51A/F56V/N91R/K94Q/R97W-del(V105-I107) 7-Y51S/N55A/F56Q/N91R/K94Q/R97W-del(V105-I107) 8-Y51S/N55V/F56Q/N91R/K94Q/R97W-del(V105-I107) 9-Y51T/N55V/F56Q/N91R/K94Q/R97W-del(V105-I107)) with the same CsgF peptide CsgF-(1-35). Different squiggle patterns were observed in DNA translocation experiments and their signal:noise is measured. Higher accuracies can be obtained with larger signal:noise ratios.

FIGS. 26A-26B. Sequencing errors with narrow reader-heads. FIG. 26A. representation of DNA base interaction with the reader head of the CsgG pore. Approximately, 5 bases dominate the current signal at any given time when the DNA strand is translocating through the pore. FIG. 26B. Mapping plots of the signal. Event-detected signal for multiple reads mapped to modelled signal using a custom HMM, for a mixed sequence lacking homopolymer runs, and for a sequence containing three homopolymer runs of 10 T.

FIGS. 27A-27D. Mapping the reader heads of the CsgG:CsgF complex. FIG. 27A. Reader head discrimination plot for the CsgG:CsgF complex. The average variation in modelled current when the base at each read head position is varied. To calculate the read head discrimination at position i for a model of length k with alphabet of length n, we define the discrimination at read-head position las the median of the standard deviations in current level for each of the nk−1 groups of size n where position i is varied while other positions are held constant. FIG. 27B. Static DNA strands to map the reader head: A set of polyA DNA strands (SS20 to SS38) in which one base is missing from the DNA backbone (iSpc3) is created. In each strand, the position of iSpc3 moves from 3′ end towards the 5′ end. Based on previous experiments with the CsgG pore, 7^(th) position of the DNA is expected to be located within the CsgG constriction. SS26 corresponds to this DNA is highlighted. Based on the model from FIG. 27A, 4-5 bases are expected to separate CsgG and CsgF reader heads. Therefore, approximately, position 12 and 13 are expected to be within the CsgF constriction. SS31 and SS32 DNA strands corresponding to those positions are highlighted. FIGS. 27C-27D. Mapping the two reader heads: Biotin modification at the 3′ end of each strand is complexed with monovalent streptavidin and the current blockage generated from each strand is recorded in a MinION set up. When the iSpc3 position is present above or below the constriction within the pore, no deflection is expected. However, when the iSpc3 is located within the constriction, a higher current level is expected to pass through the pore—the extra space created by the lack of base lets more ions to pass through. Therefore, by plotting the current passing through with each DNA strand, the locations of the two reader heads can be mapped. As expected, the highest deflection in the current is seen when the position 7 of the DNA strand is occupied by iSpc3 (FIG. 27C). iSpc3 at positions 6 and 8 also produce a higher deflection over the average polyA current level. Therefore, positions 6, 7 and 8 of the DNA strand represent the first reader head—CsgG reader head. As expected, when positions 12^(th) and 13^(th) are occupied by iCsp3, another deviation from baseline polyA is observed (FIG. 27D). This indicates the second reader head of the pore—CsgF reader head. Results also confirm that the two reader heads are apart by approximately 4-5 bases.

FIGS. 28A-28B. Reader head discrimination and base contribution. Left hand panel demonstrates the read-head discrimination of each mutant pore: the average variation in modelled current when the base at each read head position is varied. To calculate the read head discrimination at position i for a model of length k with alphabet of length n, we define the discrimination at read-head position i as the median of the standard deviations in current level for each of the n^(k-1) groups of size n where position i is varied while other positions are held constant. Right hand panel demonstrates the base contribution plot: Median current over all sequence contexts with base b (A, T, G or C) at position i of the reader head.

FIGS. 29A-29C. Error profiles of the double reader head pore. FIG. 29A. Schematic representation of the CsgG:CsgF complex and the interaction of bases of the DNA with the two reader heads. Red: strong interactions, orange: weak interactions, grey: no interactions. FIG. 29B. Comparison of errors in deletions. Reads from Y51A/F56Q/N91R/K94Q/R97W/R192D-del(V105-I107) and Y51A/N55V/F56Q/N91R/K94Q/R97W-del(V105-I107): CsgF-N17S-(1-35) pores were base called from the same region of E. coli DNA. Reads were aligned to the reference genome using Minimap2 (arxiv.org/abs/1708.01492), and the resultant alignments were visualised in Savant Genome Browser (www.ncbi.nlm.nih.gov/pubmed/20562449). The majority of Y51A/F56Q/N91R/K94Q/R97W/R192D-del(V105-I107) reads contain a single base deletion (black boxes) in the T homopolymer, which is not present in the majority of CsgG:CsgF reads. FIG. 29C. Comparison of the consensus accuracy from unpolished data generated from Y51A/F56Q/N91R/K94Q/R97W/R192D-del(V105-I107) (blue) and Y51A/N55V/F56Q/N91R/K94Q/R97W-del(V105-I107):CsgF-N17S-(1-35) pores (green) against the length of homopolymers.

FIGS. 30A-30H. Homopolymer calling of CsgG:CsgF complex. FIG. 30A. DNA with the sequence shown in (A) is translocated through the Y51A/F56Q/N91R/K94Q/R97W/R192D-del(V105-I107) pore (B) and the Y51A/N55V/F56Q/N91R/K94Q/R97W-del(V105-I107):CsgF-N17S-(1-35) pore (C) and their signal was analysed for the first polyT section shown in red in (A). FIGS. 30B-30H. When the polyT section is passing through the CsgG pore which contains a single reader head (model is based on 5 bases located in the reader head), it generates a flat line in the signal. Therefore, it is difficult to determine the exact number of bases in this region which usually causes deletion errors. When the DNA is passing through the CsgG:CsgF complex which contains two reader heads (model is based on 9 bases located within and in between the two reader heads), polyT section shows multiple steps instead of a flat line. Information in these steps can be used to correctly identify the number of bases in the homopolymeric region. This additional information significantly reduce deletion errors and improves overall consensus accuracy.

FIGS. 31A-31C. Characterisation of the CsgG pore (Y51A/F56Q/N91R/K94Q/R97W/-del(V105-I107). FIG. 31A. Reader head discrimination of the CsgG pore. The average variation in modelled current when the base at each read head position is varied. To calculate the read head discrimination at position i for a model of length k with alphabet of length n, we define the discrimination at read-head position i as the median of the standard deviations in current level for each of the n^(k-1) groups of size n where position i is varied while other positions are held constant. FIG. 31B. Base contribution plot of the CsgG pore. Median current over all k mers with base b (A, T, G or C) at position l of the reader head. FIG. 31C. Current signature when the DNA strand is passing through the CsgG pore.

DETAILED DESCRIPTION

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. Any reference signs in the claims shall not be construed as limiting the scope. Of course, it is to be understood that not necessarily all aspects or advantages may be achieved in accordance with any particular embodiment of the invention. Thus, for example those skilled in the art will recognize that the invention may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other aspects or advantages as may be taught or suggested herein.

The invention, both as to organization and method of operation, together with features and advantages thereof, may best be understood by reference to the following detailed description when read in conjunction with the accompanying drawings. The aspects and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter. Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment, but may. Similarly, it should be appreciated that in the description of exemplary embodiments of the invention, various features of the invention are sometimes grouped together in a single embodiment, figure, or description thereof for the purpose of streamlining the disclosure and aiding in the understanding of one or more of the various inventive aspects. This method of disclosure, however, is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment.

In addition as used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. Thus, for example, reference to “a polynucleotide” includes two or more polynucleotides, reference to “a polynucleotide binding protein” includes two or more such proteins, reference to “a helicase” includes two or more helicases, reference to “a monomer” refers to two or more monomers, reference to “a pore” includes two or more pores and the like.

In all of the discussion herein, the standard one letter codes for amino acids are used. These are as follows: alanine (A), arginine (R), asparagine (N), aspartic acid (D), cysteine (C), glutamic acid (E), glutamine (Q), glycine (G), histidine (H), isoleucine (I), leucine (L), lysine (K), methionine (M), phenylalanine (F), proline (P), serine (S), threonine (T), tryptophan (W), tyrosine (Y) and valine (V). Standard substitution notation is also used, i.e. Q42R means that Q at position 42 is replaced with R.

In the paragraphs herein where different amino acids at a specific position are separated by the / symbol, the / symbol means “or”. For instance, Q87R/K means Q87R or Q87K.

In the paragraphs herein where different positions are separated by the / symbol, the / symbol means “and” such that Y51/N55 is Y51 and N55.

All amino-acid substitutions, deletions and/or additions disclosed herein are with reference to a mutant CsgG monomer comprising a variant of the sequence shown in SEQ ID NO: 3, unless stated to the contrary.

Reference to a mutant CsgG monomer comprising a variant of the sequence shown in SEQ ID NO: 3 encompasses mutant CsgG monomers comprising variants of sequences as set out in the further SEQ ID NOS as disclosed below. Amino-acid substitutions, deletions and/or additions may be made to CsgG monomers comprising a variant of the sequence other than shown in SEQ ID NO: 3 that are equivalent to those substitutions, deletions and/or additions disclosed herein with reference to a mutant CsgG monomer comprising a variant of the sequence shown in SEQ ID NO: 3.

All publications, patents and patent applications cited herein, whether supra or infra, are hereby incorporated by reference in their entirety.

Definitions

Where an indefinite or definite article is used when referring to a singular noun e.g. “a” or “an”, “the”, this includes a plural of that noun unless something else is specifically stated. Where the term “comprising” is used in the present description and claims, it does not exclude other elements or steps. Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein. The following terms or definitions are provided solely to aid in the understanding of the invention. Unless specifically defined herein, all terms used herein have the same meaning as they would to one skilled in the art of the present invention. Practitioners are particularly directed to Sambrook et al., Molecular Cloning: A Laboratory Manual, 4^(th) ed., Cold Spring Harbor Press, Plainsview, N.Y. (2012); and Ausubel et al., Current Protocols in Molecular Biology (Supplement 114), John Wiley & Sons, New York (2016), for definitions and terms of the art. The definitions provided herein should not be construed to have a scope less than understood by a person of ordinary skill in the art.

“About” as used herein when referring to a measurable value such as an amount, a temporal duration, and the like, is meant to encompass variations of ±20% or +10%, more preferably ±5%, even more preferably ±1%, and still more preferably ±0.1% from the specified value, as such variations are appropriate to perform the disclosed methods.

“Nucleotide sequence”, “DNA sequence” or “nucleic acid molecule(s)” as used herein refers to a polymeric form of nucleotides of any length, either ribonucleotides or deoxyribonucleotides. This term refers only to the primary structure of the molecule. Thus, this term includes double- and single-stranded DNA, and RNA. The term “nucleic acid” as used herein, is a single or double stranded covalently-linked sequence of nucleotides in which the 3′ and 5′ ends on each nucleotide are joined by phosphodiester bonds. The polynucleotide may be made up of deoxyribonucleotide bases or ribonucleotide bases. Nucleic acids may be manufactured synthetically in vitro or isolated from natural sources. Nucleic acids may further include modified DNA or RNA, for example DNA or RNA that has been methylated, or RNA that has been subject to post-translational modification, for example 5′-capping with 7-methylguanosine, 3′-processing such as cleavage and polyadenylation, and splicing. Nucleic acids may also include synthetic nucleic acids (XNA), such as hexitol nucleic acid (HNA), cyclohexene nucleic acid (CeNA), threose nucleic acid (TNA), glycerol nucleic acid (GNA), locked nucleic acid (LNA) and peptide nucleic acid (PNA). Sizes of nucleic acids, also referred to herein as “polynucleotides” are typically expressed as the number of base pairs (bp) for double stranded polynucleotides, or in the case of single stranded polynucleotides as the number of nucleotides (nt). One thousand bp or nt equal a kilobase (kb). Polynucleotides of less than around 40 nucleotides in length are typically called “oligonucleotides” and may comprise primers for use in manipulation of DNA such as via polymerase chain reaction (PCR).

“Gene” as used here includes both the promoter region of the gene as well as the coding sequence. It refers both to the genomic sequence (including possible introns) as well as to the cDNA derived from the spliced messenger, operably linked to a promoter sequence.

“Coding sequence” is a nucleotide sequence, which is transcribed into mRNA and/or translated into a polypeptide when placed under the control of appropriate regulatory sequences. The boundaries of the coding sequence are determined by a translation start codon at the 5′-terminus and a translation stop codon at the 3′-terminus. A coding sequence can include, but is not limited to mRNA, cDNA, recombinant nucleotide sequences or genomic DNA, while introns may be present as well under certain circumstances.

The term “amino acid” in the context of the present disclosure is used in its broadest sense and is meant to include organic compounds containing amine (NH₂) and carboxyl (COOH) functional groups, along with a side chain (e.g., a R group) specific to each amino acid. In some embodiments, the amino acids refer to naturally occurring L α-amino acids or residues. The commonly used one and three letter abbreviations for naturally occurring amino acids are used herein: A=Ala; C=Cys; D=Asp; E=Glu; F=Phe; G=Gly; H=His; I=Ile; K=Lys; L=Leu; M=Met; N=Asn; P=Pro; Q=Gln; R=Arg; S=Ser; T=Thr; V=Val; W=Trp; and Y=Tyr (Lehninger, A. L., (1975) Biochemistry, 2d ed., pp. 71-92, Worth Publishers, New York). The general term “amino acid” further includes D-amino acids, retro-inverso amino acids as well as chemically modified amino acids such as amino acid analogues, naturally occurring amino acids that are not usually incorporated into proteins such as norleucine, and chemically synthesised compounds having properties known in the art to be characteristic of an amino acid, such as β-amino acids. For example, analogues or mimetics of phenylalanine or proline, which allow the same conformational restriction of the peptide compounds as do natural Phe or Pro, are included within the definition of amino acid. Such analogues and mimetics are referred to herein as “functional equivalents” of the respective amino acid. Other examples of amino acids are listed by Roberts and Vellaccio, The Peptides: Analysis, Synthesis, Biology, Gross and Meiehofer, eds., Vol. 5 p. 341, Academic Press, Inc., N.Y. 1983, which is incorporated herein by reference.

The terms “protein”, “polypeptide”, and “peptide” are interchangeably used further herein to refer to a polymer of amino acid residues and to variants and synthetic analogues of the same. Thus, these terms apply to amino acid polymers in which one or more amino acid residues is a synthetic non-naturally occurring amino acid, such as a chemical analogue of a corresponding naturally occurring amino acid, as well as to naturally-occurring amino acid polymers. Polypeptides can also undergo maturation or post-translational modification processes that may include, but are not limited to: glycosylation, proteolytic cleavage, lipidization, signal peptide cleavage, propeptide cleavage, phosphorylation, and such like. By “recombinant polypeptide” is meant a polypeptide made using recombinant techniques, e.g., through the expression of a recombinant or synthetic polynucleotide. When the chimeric polypeptide or biologically active portion thereof is recombinantly produced, it is also preferably substantially free of culture medium, e.g., culture medium represents less than about 20%, more preferably less than about 10%, and most preferably less than about 5% of the volume of the protein preparation. By “isolated” is meant material that is substantially or essentially free from components that normally accompany it in its native state. For example, an “isolated polypeptide”, as used herein, refers to a polypeptide, which has been purified from the molecules which flank it in a naturally-occurring state, e.g., a protein complex or CsgF peptide which has been removed from the molecules present in the production host that are adjacent to said polypeptide. An isolated CsgF peptide (optionally a truncated CsgF peptide) can be generated by amino acid chemical synthesis or can be generated by recombinant production. An isolated complex can be generated by in vitro reconstitution after purification of the components of the complex, e.g. a CsgG pore and the CsgF peptide(s), or can be generated by recombinant co-expression.

“Orthologues” and “paralogues” encompass evolutionary concepts used to describe the ancestral relationships of genes. Paralogues are genes within the same species that have originated through duplication of an ancestral gene; orthologues are genes from different organisms that have originated through speciation, and are also derived from a common ancestral gene.

“Homologue”, “Homologues” of a protein encompass peptides, oligopeptides, polypeptides, proteins and enzymes having amino acid substitutions, deletions and/or insertions relative to the unmodified or wild-type protein in question and having similar biological and functional activity as the unmodified protein from which they are derived. The term “amino acid identity” as used herein refers to the extent that sequences are identical on an amino acid-by-amino acid basis over a window of comparison. Thus, a “percentage of sequence identity” is calculated by comparing two optimally aligned sequences over the window of comparison, determining the number of positions at which the identical amino acid residue (e.g., Ala, Pro, Ser, Thr, Gly, Val, Leu, lie, Phe, Tyr, Trp, Lys, Arg, His, Asp, Glu, Asn, Gln, Cys and Met) occurs in both sequences to yield the number of matched positions, dividing the number of matched positions by the total number of positions in the window of comparison (i.e., the window size), and multiplying the result by 100 to yield the percentage of sequence identity.

The term “CsgG pore” defines a pore comprising multiple CsgG monomers. Each CsgG monomer may be a wild-type monomer from E. coli (SEQ ID NO: 3), wild-type homologues of E. coli CsgG, such as for example, monomers having any one of the amino acid sequences shown in SEQ ID NOS: 68 to 88. or a variant of any thereof (e.g. a variant of any one of SEQ ID NOs: 3 and 68 to 88). The variant CsgG monomer may also be referred to as a modified CsgG monomer or a mutant CsgG monomer. The modifications, or mutations, in the variant include but are not limited to any one or more of the modifications disclosed herein, or combinations of said modifications.

For all aspects and embodiments of the present invention, a CsgG homologue is referred to as a polypeptide that has at least 50%, 60%, 70%, 80%, 90%, 95% or 99% complete sequence identity to wild-type E. coli CsgG as shown in SEQ ID NO: 3. A CsgG homologue is also referred to as a polypeptide that contains the PFAM domain PF03783, which is characteristic for CsgG-like proteins. A list of presently known CsgG homologues and CsgG architectures can be found at pfam.xfam.orq//family/PF03783. Likewise, a CsgG homologous polynucleotide can comprise a polynucleotide that has at least 50%, 60%, 70%, 80%, 90%, 95% or 99% complete sequence identity to wild-type E. coli CsgG as shown in SEQ ID NO: 1. Examples of homologues of CsgG shown in SEQ ID NO:3 have the sequences shown in SEQ ID NOS: 68 to 88.

The term “modified CsgF peptide” or “CsgF peptide” defines CsgF peptide that has been truncated from its C-terminal end (e.g. is an N-terminal fragment) and/or is modified to include a cleavage site. The CsgF peptide may be a fragment of wild-type E. coli CsgF (SEQ ID NO: 5 or SEQ ID NO: 6), or of a wild-type homologue of E. coli CsgF, such as for example, a peptide comprising any one of the amino acid sequences shown in SEQ ID NOS: 17 to 36. or a variant (e.g. one modified to include a cleavage site) of any thereof.

For all aspects and embodiments of the present invention, a CsgF homologue is referred to as a polypeptide that has at least 50%, 60%, 70%, 80%, 90%, 95% or 99% complete sequence identity to wild-type E. coli CsgF as shown in SEQ ID NO: 6. In some embodiments, a CsgF homologue is also referred to as a polypeptide that contains the PFAM domain PF 0614, which is characteristic for CsgF-like proteins. A list of presently known CsgF homologues and CsgF architectures can be found at pfam.xfam.orq//family/PF10614. Likewise, a CsgF homologous polynucleotide can comprise a polynucleotide that has at least 50%, 60%, 70%, 80%, 90%, 95% or 99% complete sequence identity to wild-type E. coli CsgF as shown in SEQ ID NO: 4. Examples of truncated regions of homologues of CsgF shown in SEQ ID NO:6 have the sequences shown in SEQ ID NOs:17 to 36.

The term “N-terminal portion of a CsgF mature peptide” refers to a peptide having an amino acid sequence that corresponds to the first 60, 50, or 40 amino acid residues starting from the N-terminus of a CsgF mature peptide (without a signal sequence). The CsgF mature peptide can be a wild-type or mutant (e.g., with one or more mutations).

Sequence identity can also be to a fragment or portion of the full length polynucleotide or polypeptide. Hence, a sequence may have only 50% overall sequence identity with a full length reference sequence, but a sequence of a particular region, domain or subunit could share 80%, 90%, or as much as 99% sequence identity with the reference sequence. Homology to the nucleic acid sequence of SEQ ID NO: 1 for CsgG homologues or SEQ ID NO:4 for CsgF homologues, respectively, is not limited simply to sequence identity. Many nucleic acid sequences can demonstrate biologically significant homology to each other despite having apparently low sequence identity. Homologous nucleic acid sequences are considered to be those that will hybridise to each other under conditions of low stringency (M. R. Green, J. Sambrook, 2012, Molecular Cloning: A Laboratory Manual, Fourth Edition, Books 1-3, Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.).

The term “wild-type” refers to a gene or gene product isolated from a naturally occurring source. A wild-type gene is that which is most frequently observed in a population and is thus arbitrarily designed the “normal” or “wild-type” form of the gene. In contrast, the term “modified”, “mutant” or “variant” refers to a gene or gene product that displays modifications in sequence (e.g., substitutions, truncations, or insertions), post-translational modifications and/or functional properties (e.g., altered characteristics) when compared to the wild-type gene or gene product. It is noted that naturally occurring mutants can be isolated; these are identified by the fact that they have altered characteristics when compared to the wild-type gene or gene product. Methods for introducing or substituting naturally-occurring amino acids are well known in the art. For instance, methionine (M) may be substituted with arginine (R) by replacing the codon for methionine (ATG) with a codon for arginine (CGT) at the relevant position in a polynucleotide encoding the mutant monomer. Methods for introducing or substituting non-naturally-occurring amino acids are also well known in the art. For instance, non-naturally-occurring amino acids may be introduced by including synthetic aminoacyl-tRNAs in the IVTT system used to express the mutant monomer. Alternatively, they may be introduced by expressing the mutant monomer in E. coli that are auxotrophic for specific amino acids in the presence of synthetic (i.e. non-naturally-occurring) analogues of those specific amino acids. They may also be produced by naked ligation if the mutant monomer is produced using partial peptide synthesis. Conservative substitutions replace amino acids with other amino acids of similar chemical structure, similar chemical properties or similar side-chain volume. The amino acids introduced may have similar polarity, hydrophilicity, hydrophobicity, basicity, acidity, neutrality or charge to the amino acids they replace. Alternatively, the conservative substitution may introduce another amino acid that is aromatic or aliphatic in the place of a pre-existing aromatic or aliphatic amino acid. Conservative amino acid changes are well-known in the art and may be selected in accordance with the properties of the 20 main amino acids as defined in Table 1 below. Where amino acids have similar polarity, this can also be determined by reference to the hydropathy scale for amino acid side chains in Table 2.

TABLE 1 Chemical properties of amino acids Ala aliphatic, hydrophobic, neutral Met hydrophobic, neutral Cys polar, hydrophobic, neutral Asn polar, hydrophilic, neutral Asp polar, hydrophilic, charged (−) Pro hydrophobic, neutral Glu polar, hydrophilic, charged (−) Gln polar, hydrophilic, neutral Phe aromatic, hydrophobic, neutral Arg polar, hydrophilic, charged (+) Gly aliphatic, neutral Ser polar, hydrophilic, neutral His aromatic, polar, hydrophilic, charged (+) Thr polar, hydrophilic, neutral Ile aliphatic, hydrophobic, neutral Val aliphatic, hydrophobic, neutral Lys polar, hydrophilic, charged(+) Trp aromatic, hydrophobic, neutral Leu aliphatic, hydrophobic, neutral Tyr aromatic, polar, hydrophobic

TABLE 2 Hydropathy scale Side Chain Hydropathy Ile 4.5 Val 4.2 Leu 3.8 Phe 2.8 Cys 2.5 Met 1.9 Ala 1.8 Gly −0.4 Thr −0.7 Ser −0.8 Trp −0.9 Tyr −1.3 Pro −1.6 His −3.2 Glu −3.5 Gln −3.5 Asp −3.5 Asn −3.5 Lys −3.9 Arg −4.5

A mutant or modified protein, monomer or peptide can also be chemically modified in any way and at any site. A mutant or modified monomer or peptide is preferably chemically modified by attachment of a molecule to one or more cysteines (cysteine linkage), attachment of a molecule to one or more lysines, attachment of a molecule to one or more non-natural amino acids, enzyme modification of an epitope or modification of a terminus. Suitable methods for carrying out such modifications are well-known in the art. The mutant of modified protein, monomer or peptide may be chemically modified by the attachment of any molecule. For instance, the mutant of modified protein, monomer or peptide may be chemically modified by attachment of a dye or a fluorophore. In some embodiments, the mutant or modified monomer or peptide is chemically modified with a molecular adaptor that facilitates the interaction between a pore comprising the monomer or peptide and a target nucleotide or target polynucleotide sequence. The molecular adaptor is preferably a cyclic molecule, a cyclodextrin, a species that is capable of hybridization, a DNA binder or interchelator, a peptide or peptide analogue, a synthetic polymer, an aromatic planar molecule, a small positively-charged molecule or a small molecule capable of hydrogen-bonding.

The presence of the adaptor improves the host-guest chemistry of the pore and the nucleotide or polynucleotide sequence and thereby improves the sequencing ability of pores formed from the mutant monomer. The principles of host-guest chemistry are well-known in the art. The adaptor has an effect on the physical or chemical properties of the pore that improves its interaction with the nucleotide or polynucleotide sequence. The adaptor may alter the charge of the barrel or channel of the pore or specifically interact with or bind to the nucleotide or polynucleotide sequence thereby facilitating its interaction with the pore. Hence a modified CsgF peptide, as provided in the disclosure, may be coupled to enzymes or proteins providing better proximity of said proteins or enzymes to the pore, which may facilitate certain applications of the pore complex comprising the modified CsgF peptide.

In this context, proteins can also be fusion proteins, referring in particular to genetic fusion, made e.g., by recombinant DNA technology. Proteins can also be conjugated, or “conjugated to”, as used herein, which refers, in particular, to chemical and/or enzymatic conjugation resulting in a stable covalent link.

Proteins may form a protein complex when several polypeptides or protein monomers bind to or interact with each other. “Binding” means any interaction, be it direct or indirect. A direct interaction implies a contact between the binding partners, for instance through a covalent link or coupling. An indirect interaction means any interaction whereby the interaction partners interact in a complex of more than two compounds. The interaction can be completely indirect, with the help of one or more bridging molecules, or partly indirect, where there is still a direct contact between the partners, which is stabilized by the additional interaction of one or more compounds. The “complex” as referred to in this disclosure is defined as a group of two or more associated proteins, which might have different functions. The association between the different polyptides of the protein complex might be via non-covalent interactions, such as hydrophobic or ionic forces, or may as well be a covalent binding or coupling, such as disulphide bridges, or peptidic bounds. Covalent “binding” or “coupling” are used interchangeably herein, and may also involve “cysteine coupling” or “reactive or photoreactive amino acid coupling”, referring to a bioconjugation between cysteines or between (photo)reactive amino acids, respectively, which is a chemical covalent link to form a stable complex. Examples of photoreactive amino acids include azidohomoalanine, homopropargylglycyine, homoallelglycine, p-acetyl-Phe, p-azido-Phe, p-propargyloxy-Phe and p-benzoyl-Phe (Wang et al. 2012, in Protein Engineering, DOI: 10.5772/28719; Chin et al. 2002, Proc. Nat. Acad. Sci. USA 99(17); 11020-24).

A “biological pore” is a transmembrane protein structure defining a channel or hole that allows the translocation of molecules and ions from one side of the membrane to the other. The translocation of ionic species through the pore may be driven by an electrical potential difference applied to either side of the pore. A “nanopore” is a biological pore in which the minimum diameter of the channel through which molecules or ions pass is in the order of nanometres (10⁻⁹ metres). In some embodiments, the biological pore can be a transmembrane protein pore. The transmembrane protein structure of a biological pore may be monomeric or oligomeric in nature. Typically, the pore comprises a plurality of polypeptide subunits arranged around a central axis thereby forming a protein-lined channel that extends substantially perpendicular to the membrane in which the nanopore resides. The number of polypeptide subunits is not limited. Typically, the number of subunits is from 5 to up to 30, suitably the number of subunits is from 6 to 10. Alternatively, the number of subunits is not defined as in the case of perfringolysin or related large membrane pores. The portions of the protein subunits within the nanopore that form protein-lined channel typically comprise secondary structural motifs that may include one or more trans-membrane β-barrel, and/or α-helix sections.

The term “pore”, “pore complex”, or “complex pore”, as used interchangeably herein, refer to an oligomeric pore, wherein for instance at least a CsgG monomer (including, e.g., one or more CsgG monomers such as two or more CsgG monomers, three or more CsgG monomers) or a CsgG pore (comprised of CsgG monomers), and a CsgF peptide (e.g., a modified or truncated CsgF peptide) are associated in the complex and together form a pore or a nanopore. The pore complex of the disclosure has the features of a biological pore, i.e. it has a typical transmembrane protein structure. When the pore complex is provided in an environment having membrane components, membranes, cells, or an insulating layer, the pore complex will insert in the membrane or the insulating layer, and form a “transmembrane pore complex”.

The pore complex or transmembrane pore complex of the disclosure is suited for analyte characterization. In some embodiments, the pore complex or transmembrane complex described herein can be used for sequencing polynucleotide sequences e.g., because it can discriminate between different nucleotides with a high degree of sensitivity. The pore complex of the disclosure may be an isolated pore complex, substantially isolated, purified or substantially purified. A pore complex of the disclosure is “isolated” or purified if it is completely free of any other components, such as lipids or other pores, or other proteins with which it is normally associated in its native state e.g., CsgE, CsgA CsgB, or if it is sufficiently enriched from a membranous compartment. A pore complex is substantially isolated if it is mixed with carriers or diluents which will not interfere with its intended use. For instance, a pore complex is substantially isolated or substantially purified if it is present in a form that comprises less than 10%, less than 5%, less than 2% or less than 1% of other components, such as triblock copolymers, lipids or other pores. Alternatively, a pore complex of the disclosure may be a transmembrane pore complex, when present in a membrane. The disclosure provides isolated pore complexes comprising a homo-oligomeric pore derived from CsgG comprising identical mutant monomers, which may also contain a mutant form of the CsgG monomer, as a homologue thereof. Alternatively, an isolated pore complex comprising a hetero-oligomeric CsgG pore is provided, which can be CsgG pore consisting of mutant and wild-type CsgG monomers, or of different forms of CsgG variants, mutants or homologues. The isolated pore complex typically comprises at least 7, at least 8, at least 9 or at least 10 CsgG monomers, and 1 or more (modified) CsgF peptides, such as 2, 3, 4, 5, 6, 7, 8, 9, 10 CsgF peptides. The pore complex may comprise any ratio of CsG monomer:CsgF peptide. In one embodiment, the ratio of CsG monomer:CsgF peptide is 1:1.

The “constriction”, “orifice”, “constriction region”, “channel constriction”, or “constriction site”, as used interchangeably herein, refers to an aperture defined by a luminal surface of a pore or pore complex, which acts to allow the passage of ions and target molecules (e.g., but not limited to polynucleotides or individual nucleotides) but not other non-target molecules through the pore complex channel. In some embodiments, the constriction(s) are the narrowest aperture(s) within a pore or pore complex. In this embodiment, the constriction(s) may serve to limit the passage of molecules through the pore. The size of the constriction is typically a key factor in determining suitability of a nanopore for nucleic acid sequencing applications. If the constriction is too small, the molecule to be sequenced will not be able to pass through. However, to achieve a maximal effect on ion flow through the channel, the constriction should not be too large. For example, the constriction should not be wider than the solvent-accessible transverse diameter of a target analyte. Ideally, any constriction should be as close as possible in diameter to the transverse diameter of the analyte passing through. For sequencing of nucleic acids and nucleic acid bases, suitable constriction diameters are in the nanometre range (10⁻⁹ meter range). Suitably, the diameter should be in the region of 0.5 to 2.0 nm, typically, the diameter is in the region of 0.7 to 1.2 nm. The constriction in wild type E. coli CsgG has a diameter of approximately 9 A (0.9 nm). The CsgF constriction formed in the pore complex comprising the CsgG-like pore and the modified CsgF peptide, or homologues or mutants thereof, has a diameter in the range of 0.5 to 2 nm or in the range of 0.7 to 1.2 nm and is hence suitable for nucleic acid sequencing.

When two or more constrictions are present and spaced apart each constriction may interact or “read” separate nucleotides within the nucleic acid strand at the same time. In this situation, the reduction in ion flow through the channel will be the result of the combined restriction in flow of all the constrictions containing nucleotides. Hence, in some instances a double constriction may lead to a composite current signal. In certain circumstances, the current read-out for one constriction, or “reading head”, may not be able to be determined individually when two such reading heads are present. The constriction of wildtype E. coli CsgG (SEQ ID NO:3) is composed of two annular rings formed by juxtaposition of tyrosine residues at position 51 (Tyr 51) in the adjacent protein monomers, and also the phenylalanine and asparagine residues at positions 56 and 55 respectively (Phe 56 and Asn 55) (FIG. 1). The wild-type pore structure of CsgG is in most cases being re-engineered via recombinant genetic techniques to widen, alter, or remove one of the two annular rings that make up the CsgG constriction (mentioned as “CsgG channel constriction” herein), to leave a single well-defined reading head. The constriction motif in the CsgG oligomeric pore is located at amino acid residues at position 38 to 63 in the wild type monomeric E. coli CsgG polypeptide, depicted in SEQ ID NO: 3. In considering this region, mutations at any of the amino acid residue positions 50 to 53, 54 to 56 and 58 to 59, as well as key of positioning of the sidechains of Tyr51, Asn55, and Phe56 within the channel of the wild-type CsgG structure, was shown to be advantageous in order to modify or alter the characteristics of the reading head. The present disclosure relating to a pore complex comprising a CsgG-pore and a modified CsgF peptide, or homologues or mutants thereof, surprisingly added another constriction (mentioned as “CsgF channel constriction” herein) to the CsgG-containing pore complex, forming a suitable additional, second reader head in the pore, via complex formation with the modified CsgF peptide. Said additional CsgF channel constriction or reader head is positioned adjacent to the constriction loop of the CsgG pore, or of the mutated GcsG pore. Said additional CsgF channel constriction or reader head is positioned approximately 10 nm or less, such as 5 nm or less, such as 1, 2, 3, 4, 5, 6, 7, 8, 9 nm from the constriction loop of the CsgG pore, or of the mutated GcsG pore. The pore complex or transmembrane pore complex of the disclosure includes pore complexes with two reader heads, meaning, channel constrictions positioned in such a way to provide a suitable separate reader head without interfering the accuracy of other constriction channel reader heads. Said pore complexes therefore may include CsgG mutant pores (see incorporated references WO2016/034591, WO2017/149316, WO2017/149317, WO2017/149318 and International patent application no. PCT/GB2018/051191 each of which lists mutations to the wild-type CsgG pore that improve the properties of the pore) as well as wild-type CsgG pores, or homologues thereof, together with a modified CsgF peptide, or homologue or mutant thereof, wherein said CsgF peptide has another constriction channel forming a reader head.

Pores

The invention relates to CsgG pores complexed with an extracellularly located CsgF peptide that surprisingly introduces an additional channel constriction or reader head in the pore complex. Moreover, the disclosure provides positional information for the constriction made by the CsgF peptide within the pore complex, the peptide being inserted in the lumen of the CsgG pore, and the constriction site being in the N-terminal part of the CsgF protein. Furthermore, modified or truncated CsgF peptides of the disclosure were shown to be sufficient for pore complex formation, and provide means and methods for biosensing applications. The disclosure comprises wildtype and mutant CsgG pores (as disclosed in e.g., WO2016/034591, WO2017/149316, WO2017/149317, WO2017/149318 and International patent application no. PCT/GB2018/051191), or homologues or mutants thereof, combined with modified or truncated CsgF peptides and its mutants or homologues, all together improving the ability of the CsgG-like pore complex to interact with an analyte, such as a polynucleotide. The additional constriction introduced in the CsgG-like nanopore channel by complex formation with (modified or truncated) CsgF peptides expands the contact surface with passing analytes and can act as a second reader head for analyte detection and characterization. Pores comprising mutant CsgG monomers combined with novel mutant or modified forms of CsgF can improve the characterisation of analytes, such as polynucleotides, providing a more discriminating direct relationship between the observed current as the polynucleotide moves through the pore. In particular, by having two stacked reader heads spaced at a defined distance, the CsgG:CsgF pore complex may facilitate characterization of polynucleotides that contain at least one homopolymeric stretch, e.g., several consecutive copies of the same nucleotide that otherwise exceed the interaction length of the single CsgG reader head. Additionally, by having two stacked constrictions at a defined distance, small molecule analytes including organic or inorganic drugs and pollutants passing through the CsgG:CsgF complex pore will consecutively pass two independent reader heads. The chemical nature of either reader head can be independently modified, each giving unique interaction properties with the analyte, thus providing additional discriminating power during analyte detection.

In a first aspect, the invention relates to an isolated pore complex, comprising a CsgG pore, or a homologue or mutant thereof, or a CsgG-like pore, and a modified CsgF peptide, or a homologue or mutant thereof. In fact, the disclosure relates to a modified CsgG biological pore, comprising a modified CsgF peptide, which can be a truncated, mutant and/or variant thereof. In one embodiment, the interaction region between said modified CsgF peptide or homologue or mutant thereof, is located at the lumen of the CsgG pore, or its homologues or mutants. In another embodiment, the pore complex has two or more constriction sites or reader heads, provided by at least one constriction of the CsgG pore, and by at least one being introduced by the CsgF peptide, forming a complex with the CsgG pore. N-terminal CsgF positions with the inclusion of positions in the range of amino acid residues 39-64 of SEQ ID NO:5, or more particularly of amino acid residues 49-64 of SEQ ID NO:5, were shown to allow detectable amounts of a stable CsgG:CsgF complex. In one embodiment, the CsgF constriction produced by a modified CsgF peptide (e.g., the ones described herein) is adjacent or head-to-head of the first constriction in the CsgG pore of the pore complex. For CsgG or CsgG-like protein pores, the constriction site has been determined to be formed by a loop region of a beta strand (see FIG. 1).

In one embodiment, the modified CsgF peptide is a peptide wherein said modification in particular refers to a truncated CsgF protein or fragment, comprising an N-terminal CsgF peptide fragment defined by the limitation to contain the constriction region and to bind CsgG monomers, or homologues or mutants thereof. Said modified CsgF peptide may additionally comprise mutations or homologous sequences, which may facilitate certain properties of the pore complex. In a particular embodiment, modified CsgF peptides comprise CsgF protein truncations as compared to the wild-type preprotein (SEQ ID NO:5) or mature protein (SEQ ID NO:6) sequence, or homologues thereof. These modified peptides are intended to function as a pore complex component introducing an additional constriction site or reader head, within the CsgG-like pore formed by CsgG and the modified or truncated CsgF peptide. Examples of truncated modified peptides are described below.

Examples of homologues of the modified CsgF peptides are for instance determined in Example 3, and reveal CsgF-like proteins or CsgF peptides comprising a homologous or similar constriction region in different bacterial strains, which may be useful in the use of similar pore complexes. The structural properties and CsgG-binding elements in the CsgF peptides derived from various CsgF homologues are conserved, such that CsgF peptides can be used in combination with different wildtype or mutant CsgG pores. This includes complexes of CsgG pores with non-cognate CsgF, meaning that the CsgG pore and the parental CsgF homologue from which the CsgF is derived do not need to originate from the same operon, bacterial species or strain.

In alternative embodiments, the CsgG pore within the pore complex is not a wild-type pore, but comprises mutations or modifications to increase pore properties as well. The isolated pore complex of the disclosure, formed by the CsgG pore, or a homologue thereof, and the modified CsgF peptide, or a homologue thereof, may be formed by the wild-type form of the CsgG pore or may be further modified in the CsgG pore, such as by directed mutagenesis of particular amino acid residues, to further enhance the desired properties of the CsgG pore for use within the pore complex. For example, in embodiments of the present invention mutations are contemplated to alter the number, size, shape, placement or orientation of the constriction within the channel. The pore complex comprising a modified mutant CsgG pore may be prepared by known genetic engineering techniques that result in the insertion, substitution and/or deletion of specific targeted amino acid residues in the polypeptide sequence. In the case of the oligomeric CsgG pore, the mutations may be made in each monomeric polypeptide subunit, or any one of the monomers, or all of the monomers. Suitably, in one embodiment of the invention the mutations described are made to all monomeric polypeptides within the oligomeric protein structure. A mutant CsgG monomer is a monomer whose sequence varies from that of a wild-type CsgG monomer and which retains the ability to form a pore. Methods for confirming the ability of mutant monomers to form pores are well-known in the art. The disclosure comprises wild type and mutant CsgG pores (e.g., as disclosed in WO2016/034591, WO2017/149316, WO2017/149317, WO2017/149318 and International patent application no. PCT/GB2018/051191), or homologues thereof, combined with modified or truncated CsgF peptides and their mutants or homologues, all together improving the ability of the CsgG-like pore complex to interact with an analyte, such as a polynucleotide. Mutant CsgG pores may comprise one or more mutant monomers. The CsgG pore may be a homopolymer comprising identical monomers, or a heteropolymer comprising two or more different monomers. The monomers may have one or more of the mutations described below in any combination.

The nanopore complex comprising a modified CsgF peptide differs as compared to the wild-type CsgF protein depicted in SEQ ID NO:6 since the modified CsgF peptide only comprises N-terminal fragments or truncates of the wild-type CsgF protein in certain embodiments. The modified CsgF peptide however may be additionally or alternatively mutated CsgF peptide in the sense that mutations as amino acid substitutions are made to allow for a better second constriction site in the pore formed by the complex comprising the CsgG pore and the modified CsgF peptide. The mutant monomers might as such have improved polynucleotide reading properties when said complex is used in nucleotide sequencing i.e. display improved polynucleotide capture and nucleotide discrimination, in addition to the improved feature of the complex to comprise two reader heads. In particular, pores constructed from the mutant peptides capture nucleotides and polynucleotides more easily than the wild type. In addition, pores constructed from the mutant peptides may display an increased current range, which makes it easier to discriminate between different nucleotides, and a reduced variance of states, which increases the signal-to-noise ratio. In addition, the number of nucleotides contributing to the current as the polynucleotide moves through pores constructed from the mutants may be decreased. This makes it easier to identify a direct relationship between the observed current as the polynucleotide moves through the pore and the polynucleotide sequence. In addition, pores constructed from the mutant peptides may display an increased throughput, e.g., are more likely to interact with an analyte, such as a polynucleotide. This makes it easier to characterise analytes using the pores. Pores constructed from the mutant peptides may insert into a membrane more easily, or may provide easier way to retain additional proteins in close vicinity of the pore complex.

In an alternative embodiment, the CsgF constriction site provided in the pore complex of the invention has a diameter in the range of 0.5 nm to 2.0 nm, thereby providing a pore complex suitable for nucleic acid sequencing, as described above.

The pore may be stabilised by covalent attachment of the CsgF peptide to the CsgG pore. The covalent linkage may for example be a disulphide bond, or click chemistry. The CsgF peptide and CsgG pore may, for example, be covalently linked via residues at a position corresponding to one or more of the following pairs of positions of SEQ ID NO: 6 and SEQ ID NO: 3, respectively: 1 and 153, 4 and 133, 5 and 136, 8 and 187, 8 and 203, 9 and 203, 11 and 142, 11 and 201, 12 and 149, 12 and 203, 26 and 191, and 29 and 144.

In the pore, the interaction between the CsgF peptide and the CsgG pore may, for example, be stabilised by hydrophobic interactions or electrostatic interactions at a position corresponding to one or more of the following pairs of positions of SEQ ID NO: 6 and SEQ ID NO: 3, respectively: 1 and 153, 4 and 133, 5 and 136, 8 and 187, 8 and 203, 9 and 203, 11 and 142, 11 and 201, 12 and 149, 12 and 203, 26 and 191, and 29 and 144.

The residues in CsgF and/or CsgG at one or more of the positions listed above may be modified in order to enhance the interaction between CsgG and CsgF in the pore.

In one embodiment, the pore of the invention may be isolated, substantially isolated, purified or substantially purified. A pore of the invention is isolated or purified if it is completely free of any other components, such as lipids or other pores. A pore is substantially isolated if it is mixed with carriers or diluents which will not interfere with its intended use. For instance, a pore is substantially isolated or substantially purified if it is present in a form that comprises less than 10%, less than 5%, less than 2% or less than 1% of other components, such as triblock copolymers, lipids or other pores. Alternatively, a pore of the invention may be present in a membrane. Suitable membranes are discussed below.

A pore of the invention may be present as an individual or single pore. Alternatively, a pore of the invention may be present in a homologous or heterologous population of two or more pores.

CsgF Peptide

A second aspect of the invention relates to novel modified CsgF monomers (peptides), or truncated CsgF proteins, or a modified or truncated peptide of a CsgF homologue or mutant. Those novel modified CsgF peptides may be used in a pore complex to integrate a second or additional reader head. Said modification or truncation is preferably resulting in a fragment of the wild-type CsgF, or of mutant or homologue CsgF protein, more preferably an N-terminal fragment.

Mature CsgF (shown in SEQ ID NO:6) can be divided into three main regions: a “CsgF constriction peptide” (FCP), a “neck” region and a “head” region (as shown in FIGS. 4 and 5). The “head” region of the CsgF peptide is distinct from a reader head of a pore as described herein. The “head” region of the CsgF peptide may also be referred to as the “C-terminal head domain”.

The FCP forms the contact region with the CsgG β-barrel, where it gives rise to an additional constriction. The neck region protrudes out of the R-barrel. In the CsgG:CsgF oligomer it forms a thin-walled hollow tube that connects the FCP to a globular head region.

Based on multiple sequence alignments (FIG. 8), co-purification experiments (FIG. 9) and the cryoEM reconstruction of the CsgG:CsgF complex at 3.4 Å resolution (FIG. 11) the CsgF constriction peptide, neck and head regions can be defined as three consecutive residue stretches in mature CsgF.

The FCP spans from approximately residues 1 to 35 of the mature CsgF (Seq ID NO:6). The FCP forms the most conserved region of the protein when comparing different CsgF orthologues (FIG. 8, FIG. 10). CryoEM 3D reconstruction shows the FCP forms a well-defined structure that binds the inside the CsgG β-barrel through non-covalent contacts with the CsgG transmembrane hairpins TM1 (residues 134 to 154 of Seq ID NO:3) and TM2 (residues 184 to 208 of Seq ID NO:3) (FIG. 1E; 11) (TM1 and TM2 are defined in Goyal P, et al., 2014). In the reconstruction, nine copies of the FCP bind the CsgG oligomer (comprising 9 monomers) and together give rise to an additional constriction approximately 2 nm above the top of CsgG constriction formed by a continuous loop spanning residues 46 to 61 of mature CsgG (Seq ID NO:3; FIG. 1E; FIG. 11).

The cryoEM 3D reconstruction also shows the CsgF N-terminal residue binds near the base or the top (depending on the orientation) of the CsgG β-barrel and exits the R-barrel at residue 32. This is in good agreement with MD simulations that show the average contact time of residue pairs in the CsgG:CsgF binding interface (Table 4). The cryoEM structure and MD simulations show residues 33-34 reside outside the CsgG β-barrel, where a well, though not strictly, conserved Pro residue (Pro 35 in Seq ID NO:6) makes the transition to the CsgF neck regions. The CsgF neck is not resolved in atomic detail in the CsgG:CsgF 3D reconstruction pointing to its conformational flexibility. The CsgF neck is predicted to span from residue 36 to approximately residue 50 (SEQ ID NO:6) based on multiple sequence alignment and secondary structure prediction. The CsgF head region forms the C-terminal part of CsgF and is predicted to span from approximately residue 51 to the CsgF C-terminus. In the CsgG:CsgF complex, this region oligomerizes to give rise to a globular structure that appears to cap the CsgG:CsgF channel (FIGS. 4, 5). Multiple sequence alignment of CsgF orthologues shows the CsgF neck to be the least conserved region and suggests it may vary in length from one orthologue to another (FIG. 8).

The CsgF peptide which forms part of the invention is a truncated CsgF peptide lacking the C-terminal head; lacking the C-terminal head and a part of the neck domain of CsgF (e.g., the truncated CsgF peptide may comprise only a portion of the neck domain of CsgF); or lacking the C-terminal head and neck domains of CsgF. The CsgF peptide may lack part of the CsgF neck domain, e.g. the CsgF peptide may comprise a portion of the neck domain, such as for example, from amino acid residue 36 at the N-terminal end of the neck domain (see SEQ ID:NO:6) (e.g. residues 36-40, 36-41, 36-42, 36-43, 36-45, 36-46 up to residues 36-50 or 36-60 of SEQ ID NO: 6). The CsgF peptide preferably comprises a CsgG-binding region and a region that forms a constriction in the pore. The CsgG-binding region typically comprises residues 1 to 8 and/or 29 to 32 of the CsgF protein (SEQ ID NO: 6 or a homologue from another species) and may include one or more modifications. The region that forms a constriction in the pore typically comprises residues 9 to 28 of the CsgF protein (SEQ ID NO: 6 or a homologue from another species) and may include one or more modifications. Residues 9 to 17 comprise the conserved motif N₉PXFGGXXX₁₇ and form a turn region. Residues 9 to 28 form an alpha-helix. X₁₇ (N17 in SEQ ID NO: 6) forms the apex of the constriction region, corresponding to the narrowest part of the CsgF constriction in the pore. The CsgF constriction region also makes stabilising contacts with the CsgG beta-barrel, primarily at residues 9, 11, 12, 18, 21 and 22 of SEQ ID NO: 6.

The CsgF peptide typically has a length of from 28 to 50 amino acids, such as 29 to 49, 30 to 45 or 32 to 40 amino acids. Preferably the CsgF peptide comprises from 29 to 35 amino acids, or 29 to 45 amino acids. The CsgF peptide comprises all or part of the FCP, which corresponds to residues 1 to 35 of SEQ ID NO: 6. Where the CsgF peptide is shorter that the FCP, the truncation is preferably made at the C-terminal end.

The CsgF fragment of SEQ ID NO:6 or of a homologue or mutant thereof may have a length of 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54 or 55 amino acids.

The CsgF peptide may comprise the amino acid sequence of SEQ ID NO: 6 from residue 1 up to any one of residues 25 to 60, such as 27 to 50, for example, 28 to 45 of SEQ ID NO: 6, or the corresponding residues from a homologue of SEQ ID NO: 6, or variant of either thereof. More specifically, the CsgF peptide may comprise SEQ ID NO: 39 (residues 1 to 29 of SEQ ID NO: 6), or a homologue or variant thereof.

Examples of such CsgF peptides comprises, consist essentially of or consist of SEQ ID NO: 15 (residues 1 to 34 of SEQ ID NO: 6), SEQ ID NO: 54 (residues 1 to 30 of SEQ ID NO: 6), SEQ ID NO: 40 (residues 1 to 45 of SEQ ID NO: 6), or SEQ ID NO: 55 (residues 1 to 35 of SEQ ID NO: 6) and homologues or variants of any thereof. Other Examples of CsgF peptides comprise, consist essentially of or consist of SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9, SEQ ID NO: 10, SEQ ID NO: 11, SEQ ID NO: 12, SEQ ID NO: 13, SEQ ID NO: 14, SEQ ID NO: 16.

In the CsgF peptide, one or more residues e.g., in SEQ ID NO: 15, SEQ ID NO: 39, SEQ ID NO: 40, SEQ ID NO: 54, or SEQ ID NO: 55 may be modified.

For example, the CsgF peptide may comprise a modification at a position corresponding to one or more of the following positions in SEQ ID NO: 6: G1, T4, F5, R8, N9, N11, F12, A26 and Q29.

The CsgF peptide may be modified to introduce a cysteine, a hydrophobic amino acid, a charged amino acid, a non-native reactive amino acid, or photoreactive amino acid, for example at a position corresponding to one or more of the following positions in SEQ ID NO: 6: G1, T4, F5, R8, N9, N11, F12, A26 and Q29.

For example, the CsgF peptide may comprise a modification at a position corresponding to one or more of the following positions in SEQ ID NO: 6: N15, N17, A20, N24 and A28. The CsgF peptide may comprise a modification at a position corresponding to D34 to stabilise the CsgG-CsgF complex. In particular embodiments, the CsgF peptide comprises one or more of the substitutions: N15S/A/T/Q/G/L/V/I/F/Y/W/R/K/D/C, N17S/A/T/Q/G/L/V/I/F/Y/W/R/K/D/C, A20S/T/Q/N/G/L/V/I/F/Y/W/R/K/D/C, N24S/T/Q/A/G/L/V/I/F/Y/W/R/K/D/C, A28S/T/Q/N/G/L/V/I/F/Y/W/R/K/D/C and D34F/Y/W/R/K/N/Q/C. The CsgF peptide may, for example, comprise one or more of the following substitutions: G1C, T4C, N17S, and D34Y or D34N.

The CsgF peptide may be produced by cleavage of a longer protein, such as full-length CsgF using an enzyme. Cleavage at a particular site may be directed by modifying the longer protein, such as full-length CsgF, to include an enzyme cleavage site at an appropriate position. Examples of CsgF amino acid sequences that have been modified to include such enzyme cleavage sites are shown in SEQ ID NOs: 56 to 67. Following cleavage all or part of the added enzyme cleavage site may be present in the CsgF peptide that associates with CsgG to form a pore. Thus the CsgF peptide may further comprise all or part of an enzyme cleavage site at its C-terminal end.

Some examples of suitable CsgF peptides are shown in Table 3 below:

TABLE 3 CsgF peptides SEQ ID Technical description Protein sequence NO: CsgF-(1-45) GTMTFQFRNPNFGGNPNNGAFLL  40 NSAQAQNSYKDPSYNDDFGIET CsgF-(1-29) GTMTFQFRNPNFGGNPNNGAFLL  39 NSAQAQ CsgF-(1-35) GTMTFQFRNPNFGGNPNNGAFLL  55 NSAQAQNSYKDP CsgF-G1C-(1-45) CTMTFQFRNPNFGGNPNNGAFLL  89 NSAQAQNSYKDPSYND DFGIET CsgF-G1C-(1-30) CTMTFQFRNPNFGGNPNNGAFLL  90 NSAQAQ CsgF-G1C-(1-35) CTMTFQFRNPNFGGNPNNGAFLL  91 NSAQAQNSYKDP CsgF-N17S-(1-35) GTMTFQFRNPNFGGNPSNGAFLL  92 NSAQAQNSYKDP CsgF-G1C/D34Y- CTMTFQFRNPNFGGNPNNGAFLL  93 (1-35) NSAQAQNSYKYP CsgF-N17S-(1-45) GTMTFQFRNPNFGGNPSNGAFLL  94 NSAQAQNSYKDPSYND DFGIET CsgF-G1C/N17S/ CTMTFQFRNPNFGGNPSNGAFLL  95 D34Y-(1-35) NSAQAQNSYKYP CsgF-G1C-(1-30)- CTMTFQFRNPNFGGNPNNGAFLL  96 H10 NSAQAQHHHHHHHHHH CsgF-G1C-(1-35)- CTMTFQFRNPNFGGNPNNGAFLL  97 H10 NSAQAQNSYKDPHHHHHHHHHH CsgF-N17S/D34Y- GTMTFQFRNPNFGGNPSNGAFLL  98 (1-35) NSAQAQNSYKYP CsgF-N17S/D34N- GTMTFQFRNPNFGGNPSNGAFLL  99 (1-35) NSAQAQNSYKNP CsgF-T4C/N17S- GTMCFQFRNPNFGGNPSNGAFLL 100 (1-35) NSAQAQNSYKDP CsgF-T4C/N17S/ GTMCFQFRNPNFGGNPSNGAFLL 101 D34Y-(1-35) NSAQAQNSYKYP CsgF-T4C/N17S/ GTMCFQFRNPNFGGNPSNGAFLL 102 D34N-(1-35) NSAQAQNSYKNP CsgF-G1C/T4C/ CTMCFQFRNPNFGGNPSNGAFLL 103 N17S-(1-35) NSAQAQNSYKDP CsgF-G1C/T4C/N17S/ CTMCFQFRNPNFGGNPSNGAFLL 104 D34N-(1-35) NSAQAQNSYKNP CsgF-G1C/T4C/N17S/ CTMCFQFRNPNFGGNPSNGAFLL 105 D34Y-(1-35) NSAQAQNSYKYP CsgF-N17V-(1-35) GTMTFQFRNPNFGGNPVNGAFLL 106 NSAQAQNSYKDP CsgF-N17A-(1-35) GTMTFQFRNPNFGGNPANGAFLL 107 NSAQAQNSYKDP CsgF-N17F-(1-35) GTMTFQFRNPNFGGNPFNGAFLL 108 NSAQAQNSYKDP CsgF-N17S-(1-30) GTMTFQFRNPNFGGNPSNGAFLL 109 NSAQAQN CsgF-N17V-(1-30) GTMTFQFRNPNFGGNPVNGAFLL 110 NSAQAQN CsgF-N17A-(1-30) GTMTFQFRNPNFGGNPANGAFLL 111 NSAQAQN CsgF-N17F-(1-30) GTMTFQFRNPNFGGNPFNGAFLL 112 NSAQAQN CsgF-N17V/D34Y- GTMTFQFRNPNFGGNPVNGAFLL 113 (1-35) NSAQAQNSYKYP CsgF-N17A/D34Y- GTMTFQFRNPNFGGNPANGAFLL 114 (1-35) NSAQAQNSYKYP CsgF-N17F/D34Y- GTMTFQFRNPNFGGNPFNGAFLL 115 (1-35) NSAQAQNSYKYP CsgF-N17S/A20Q- GTMTFQFRNPNFGGNPSNGQFLL 116 (1-35) NSAQAQNSYKDP

In particular embodiments, said CsgF fragment comprises the amino acid sequence SEQ ID NO:39, or mutant or homologue thereof. In particular, SEQ ID NO:39 comprises the first 29 amino acids of the mature CsgF peptide (SEQ ID NO:6). In another embodiment, the modified CsgF peptide of the invention is a truncated peptide comprising SEQ ID NO:40. In particular, SEQ ID NO:40 comprises the first 45 amino acids of the mature CsgF peptide (SEQ ID NO:6). In particular, the CsgF constriction site and binding site to the CsgG are located within the N-terminal CsgF peptide region, further characterised in that amino acid 39 to 64 of SEQ ID NO:5 (present in SEQ ID NO:39 and SEQ ID NO:40), or in particular amino acid 49 to 64 of SEQ ID NO:5 (present in SEQ ID NO:40, but not in SEQ ID NO:39, the latter fragment encoded by SEQ ID NO:39 showing a weaker interaction with CsgG (see Examples)), confer a higher stability to the complex. Hence, the disclosure provides a modification of the CsgF protein by truncating the protein to said peptides or peptides comprising said N-terminal fragments or constriction site region to allow complex formation with the CsgG pore, or homologues or mutants thereof, in vivo. Further limitation is provided in one embodiment relating to a modified CsgF peptide comprising SEQ ID NO:37 or SEQ ID NO:38. Finally, identification of CsgF homologous peptides, especially aligned within the constriction region (FCP peptides), also provide modified CsgF peptide homologues that may form a part of said isolated complex (e.g.; see FIGS. 8 and 10).

A further embodiment relates to the modified or truncated CsgF peptides comprising SEQ ID NO:15, wherein said SEQ ID NO:15 contains the region of the CsgF protein including several residues from the region of the CsgG binding and/or constriction site, sufficient for in vitro reconstitution of the complex pore comprising CsgG or a homologue thereof, and a modified CsgF peptide, to result in an isolated pore complex comprising a CsgF channel constriction. Another embodiment describes said modified CsgF peptide comprising SEQ ID NO:16, which contains an N-terminal fragment of the CsgF protein, and two additional amino acids (KD), which will increase solubility and stability of the (synthetic) peptide, as well to allow in vitro reconstitution of said complex pore. Further embodiments are provided wherein said modified CsgF peptide comprises SEQ ID NO:15, SEQ ID NO:16 or a homologue or mutant thereof, wherein said modified CsgF peptide is further mutated, but still retains a minimal of 35% amino acid identity to SEQ ID NO:15, or SEQ ID NO:16, respectively, within the region of the modified CsgF peptide corresponding to said SEQ ID NO:15 or 16 e.g., 40%, 50%, 60%, 70%, 80% 85%, 90% amino acid identity. Further embodiments are provided wherein said modified CsgF peptide comprises SEQ ID NO:15, SEQ ID NO:16 or a homologue or mutant thereof, wherein said modified CsgF peptide is further mutated, but still retains a minimal of 40%, 45%, 50%, 60%, 70%, 80% 85% or 90% amino acid identity to SEQ ID NO:15, or SEQ ID NO:16, respectively, within the region of the modified CsgF peptide corresponding to said SEQ ID NO:15 or 16. Those mutated regions are intended to alter and/or improve the characteristics of the CsgF constriction site, as discussed above, so for instance a more accurate target analysis can be obtained. Another embodiment discloses modified CsgF peptides wherein one or more positions in the regions comprising SEQ ID NO:39, SEQ ID NO:40, SEQ ID NO:54 or SEQ ID NO:55 are modified, and wherein said mutation(s) retain a minimal of 35% amino acid identity, or 40%, 50%, 60%, 70%, 80%, 85%, 90%, 95% amino acid identity to SEQ ID NO:39, SEQ ID NO:40, SEQ ID NO:54 or SEQ ID NO:55 in the peptide fragment corresponding to the region comprising SEQ ID NO:39, SEQ ID NO:40, SEQ ID NO:54 or SEQ ID NO:55.

So further embodiments of the invention involve the isolated pore complex comprising a CsgG pore, or homologue or mutant thereof, and modified CsgF peptide(s), or homologues or mutants thereof, wherein said modified CsgF peptide(s) are defined as described in the second aspect of the invention.

Additional embodiments relate to an isolated pore complex, wherein said CsgG pore, at least via one monomer, and the modified CsgF peptide, are coupled via covalent binding. Said covalent link or binding is in one instance possible via cysteine linkage, wherein the sulfhydryl side group of cysteine covalently links with another amino acid residue or moiety. In a second possibility, the covalent linkage is obtained via an interaction between non-native (photo)reactive amino acids. (Photo-)reactive amino acids are referring to artificial analogs of natural amino acids that can be used for crosslinking of protein complexes, and may be incorporated into proteins and peptides in vivo or in vitro. Photo-reactive amino acid analogs in common use are photoreactive diazirine analogs to leucine and methionine, and para-benzoyl-phenyl-alanine, as well as azidohomoalanine, homopropargylglycyine, homoallelglycine, p-acetyl-Phe, p-azido-Phe, p-propargyloxy-Phe and p-benzoyl-Phe (Wang et al. 2012; Chin et al. 2002). Upon exposure to ultraviolet light, they are activated and covalently bind to interacting proteins that are within a few angstroms of the photo-reactive amino acid analog. However, the positions in the CsgG monomer where said covalent linkages may take place is dependent on the exposure to the modified CsgF peptide. As shown in FIG. 1, several amino acids are in the position to provide the covalent linkage, namely positions 132, 133, 136, 138, 140, 142, 144, 145, 147, 149, 151, 153, 155, 183, 185, 187, 189, 191, 201, 203, 205, 207 or 209 of SEQ ID NO: 3, or of homologues thereof.

Another aspect of the invention relates to constructs comprising said modified CsgF peptide, wherein said peptide is covalently attached. A “construct” comprises two or more covalently attached monomers derived from modified CsgF and/or CsgG, or a homologue thereof. In other words, a construct may contain more than one monomer. In another aspect, the invention also provides a pore complex comprising at least one construct of the invention. The pore complex contains sufficient constructs and, if necessary, monomers to form the pore. For instance, an octameric pore may comprise (a) four constructs each comprising two monomers, (b) two constructs each comprising four monomers, (c) one construct comprising two monomers and six monomers that do not form part of a construct, or (d) one or two CsgF monomers in one construct, and one construct with six to seven CsgG monomers or even (e) a construct with CsgF and CsgG monomer in addition to another construct solely comprising CsgG monomers. Same and additional possibilities are provided for a nonameric pore for instance. Other combinations of constructs and monomers can be envisaged by the skilled person. One or more constructs of the invention may be used to form a pore complex for characterising, such as sequencing, polynucleotides. The construct may comprise at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9 or at least 10 monomers. The construct preferably comprises two monomers. The two or more monomers may be the same or different, may be CsgF, CsgG, CsgG/CsgF fusion monomers or homologues thereof, or any combination thereof.

Another embodiment relates to the polynucleotide or nucleic acid molecule encoding said modified CsgF peptides of the invention, or homologues or mutants thereof, or polynucleotides encoding a construct as described above.

Certain embodiments relate to an isolated transmembrane pore complex comprising the isolated pore complex according to the first and second aspect of the invention, and the components of a membrane. Said isolated transmembrane pore complex is directly applicable for use in molecular sensing, such as nucleic acid sequencing. Alternatively, a membranous composition is provided, comprising a modified CsgG/CsgF biological pore as described herein, according to the isolated pore complex of the invention, and a membrane, membrane components, or an insulating layer. One embodiment relates to an isolated transmembrane pore complex consisting of the isolated pore complex according to the invention, and the components of a membrane.

Although the CsgG:CsgF complex is very stable, when CsgF is truncated, the stability of CsgG:CsgF complexes decrease compared to a complex comprising full length CsgF. Therefore, disulphide bonds can be made between CsgG and CsgF to make the complex more stable, for example following introduction of cysteine residues at the positions identified herein. The pore complex can be made in any of the previously mentioned methods and disulphide bond formation can be induced by using oxidising agents (eg: Copper-orthophenanthroline). Other interactions (eg: hydrophobic interactions, charge-charge interactions/electrostatic interactions) can also be used in those positions instead of cysteine interactions.

In another embodiment, unnatural amino acids can also be incorporated in those positions. In this embodiment, covalent bonds made be made by via click chemistry. For example, unnatural amino acids with azide or alkyne or with a dibenzocyclooctyne (DBCO) group and/or a bicyclo[6.1.0]nonyne (BCN) group may be introduced at one or more of these positions.

Such stabilising mutations can be combined with any other modifications to CsgG and/or CsgF, for example the modifications disclosed herein.

The CsgG pore may comprise at least one, such as 2, 3, 4, 5, 6, 7, 8, 9 or 10, CsgG monomers that is/are modified to facilitate attachment to the CsgF peptide. For example a cysteine residue may be introduced at one or more of the positions corresponding to positions 132, 133, 136, 138, 140, 142, 144, 145, 147, 149, 151, 153, 155, 183, 185, 187, 189, 191, 201, 203, 205, 207 and 209 of SEQ ID NO: 3, and/or at any one of the positions identified in Table 4 as being predicted to make contact with CsgF, to facilitate covalent attachment to CsgG. As an alternative or addition to covalent attachment via cysteine residues, the pore may be stabilised by hydrophobic interactions or electrostatic interactions. To facilitate such interactions, a non-native reactive or photoreactive amino acid at a position corresponding to one or more of positions 132, 133, 136, 138, 140, 142, 144, 145, 147, 149, 151, 153, 155, 183, 185, 187, 189, 191, 201, 203, 205, 207 and 209 of SEQ ID NO: 3, and/or at any one of the positions identified in Table 4 as being predicted to make contact with CsgF.

The CsgF peptide may be modified to facilitate attachment to the CsgG pore. For example a cysteine residue may be introduced at one or more of the positions corresponding to positions 1, 4, 5, 8, 9, 11, 12, 26 or 29 of SEQ ID NO: 6, and/or at any one of the positions identified in Table 4 as being predicted to make contact with CsgF, to facilitate covalent attachment to CsgG. As an alternative or addition to covalent attachment via cysteine residues, the pore may be stabilised by hydrophobic interactions or electrostatic interactions. To facilitate such interactions, a non-native reactive or photoreactive amino acid at a position corresponding to one or more of positions 1, 4, 5, 8, 9, 11, 12, 26 or 29 of SEQ ID NO: 6, and/or at any one of the positions identified in Table 4 as being predicted to make contact with CsgF.

Preferred exemplary CsgF peptides include comprise the following mutations relative to SEQ ID NO: 6: N15X₁/N17X₂/A20X₃/N24X₄/A28X₅/D34X₆, wherein X₁ is N/S/A/T/Q/G/L/V/I/F/Y/W/R/K/D/C, X₂ is N/S/A/T/Q/G/L/V/I/F/Y/W/R/K/D/C, X₃ is A/S/T/Q/N/G/L/V/I/F/Y/W/R/K/D/C, X₄ is N/S/T/Q/A/G/L/V/I/F/Y/W/R/K/D/C, X₅ is A/S/T/Q/N/G/L/V/I/F/Y/W/R/K/D/C and X₅ is D/F/Y/W/R/K/N/Q/C. The mutations at positions N15, N17, A20, N24 and A28 are constriction mutations and the mutation at position 34 affects the interaction pf CsgF with the bottom of the CsgG pore to stabilise the interaction.

CsgG Pore

The CsgG pore may be a homo-oligomeric pore comprising identical mutant monomers of the invention. The CsgG pore may be a hetero-oligomeric pore derived from CsgG, for example comprising at least one mutant monomer as disclosed herein.

The CsgG pore may contain any number of mutant monomers. The pore typically comprises at least 7, at least 8, at least 9 or at least 10 identical mutant monomers, such as 7, 8, 9 or 10 mutant monomers. The CsgG pore preferably comprises eight or nine identical mutant monomers.

In a preferred embodiment, all of the monomers in the hetero-oligomeric CsgG pore (such as 10, 9, 8 or 7 of the monomers) are mutant monomers as disclosed herein, wherein at least one of them differs from the others. They may all differ from one another.

The mutant monomers in the CsgG pore are preferably all approximately the same length or are the same length. The barrels of the mutant monomers of the invention in the pore are preferably approximately the same length or are the same length. Length may be measured in number of amino acids and/or units of length.

A mutant monomer may be a variant of SEQ ID NO: 3. Over the entire length of the amino acid sequence of SEQ ID NO: 3, a variant will preferably be at least 50% homologous to that sequence based on amino acid identity. More preferably, the variant may be at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 80%, at least 85%, at least 90% and more preferably at least 95%, 97% or 99% homologous based on amino acid identity to the amino acid sequence of SEQ ID NO: 3 over the entire sequence. There may be at least 80%, for example at least 85%, 90% or 95%, amino acid identity over a stretch of 100 or more, for example 125, 150, 175 or 200 or more, contiguous amino acids (“hard homology”).

CsgG monomers are highly conserved (as can be readily appreciated from FIGS. 45 to 47 of WO2017/149317). Furthermore, from knowledge of the mutations in relation to SEQ ID NO: 3 it is possible to determine the equivalent positions for mutations of CsgG monomers other than that of SEQ ID NO: 3.

Thus reference to a mutant CsgG monomer comprising a variant of the sequence as shown in SEQ ID NO: 3 and specific amino-acid mutations thereof as set out in the claims and elsewhere in the specification also encompasses a mutant CsgG monomer comprising a variant of the sequence as shown in SEQ ID NOs: 68 to 88 and corresponding amino-acid mutations thereof. Likewise reference to a construct, pore or method involving the use of a pore relating to a mutant CsgG monomer comprising a variant of the sequence as shown in SEQ ID NO: 3 and specific amino-acid mutations thereof as set out in the claims and elsewhere in the specification also encompasses a construct, pore or method relating to a mutant CsgG monomer comprising a variant of the sequence according the above disclosed SEQ ID NOs and corresponding amino-acid mutations thereof. If will further be appreciated that the invention extends to other variant CsgG monomers not expressly identified in the specification that show highly conserved regions.

Standard methods in the art may be used to determine homology. For example the UWGCG Package provides the BESTFIT program which can be used to calculate homology, for example used on its default settings (Devereux et al (1984) Nucleic Acids Research 12, p 387-395). The PILEUP and BLAST algorithms can be used to calculate homology or line up sequences (such as identifying equivalent residues or corresponding sequences (typically on their default settings)), for example as described in Altschul S. F. (1993) J Mol Evol 36:290-300; Altschul, S. F et al (1990) J Mol Biol 215:403-10. Software for performing BLAST analyses is publicly available through the National Center for Biotechnology Information (www.ncbi.nlm.nih.gov/).

SEQ ID NO: 3 is the wild-type CsgG monomer from Escherichia coli Str. K-12 substr. MC4100. A variant of SEQ ID NO: 3 may comprise any of the substitutions present in another CsgG homologue. Preferred CsgG homologues are shown in SEQ ID NOs: 68 to 88. The variant may comprise combinations of one or more of the substitutions present in SEQ ID NOs: 68 to 88 compared with SEQ ID NO: 3. For example, mutations may be made at any one or more of the positions in SEQ ID NO: 3 that differ between SEQ ID NO: 3 and any one of SEQ ID NOs: 68 to 88. Such a mutation may be a substitution of an amino acid in SEQ ID NO: 3 with an amino acid from the corresponding position in any one of SEQ ID NOs: 68 to 88. Alternatively, the mutation at any one of these positions may be a substitution with any amino acid, or may be a deletion or insertion mutation, such as deletion or insertion of 1 to 10 amino acids, such as of 2 to 8 or 3 to 6 amino acids. Other than the mutations disclosed herein, the amino acids that are conserved between SEQ ID NO: 3 and all of SEQ ID NOs: 66 to 88 are preferably present in a variant of the invention. However, conservative mutations may be made at any one or more of these positions that are conserved between SEQ ID NO: 3 and all of SEQ ID NOs: 66 to 88.

The invention provides a pore-forming CsgG mutant monomer that comprises any one or more of the amino acids described herein as being substituted into a specific position of SEQ ID NO: 3 at a position in the structure of the CsgG monomer that corresponds to the specific position in SEQ ID NO: 3. Corresponding positions may be determined by standard techniques in the art. For example, the PILEUP and BLAST algorithms mentioned above can be used to align the sequence of a CsgG monomer with SEQ ID NO: 3 and hence to identify corresponding residues.

The pore-forming mutant monomer typically retains the ability to form the same 3D structure as the wild-type CsgG monomer, such as the same 3D structure as a CsgG monomer having the sequence of SEQ ID NO: 3. The 3D structure of CsgG is known in the art and is disclosed, for example, in Goyal et al (2014) Nature 516(7530):250-3. Any number of mutations may be made in the wild-type CsgG sequence in addition to the mutations described herein provided that the CsgG mutant monomer retains the improved properties imparted on it by the mutations of the present invention.

Typically the CsgG monomer will retain the ability to form a structure comprising three alpha-helicies and five beta-sheets. Mutations may be made at least in the region of CsgG which is N-terminal to the first alpha helix (which starts at S63 in SEQ ID NO:3), in the second alpha helix (from G85 to A99 of SEQ ID NO: 3), in the loop between the second alpha helix and the first beta sheet (from Q100 to N120 of SEQ ID NO: 3), in the fourth and fifth beta sheets (S173 to R192 and R198 to T107 of SEQ ID NO: 3, respectively) and in the loop between the fourth and fifth beta sheets (F193 to Q197 of SEQ ID NO: 3) without affecting the ability of the CsgG monomer to form a transmembrane pore, which transmembrane pore is capable of translocating polypeptides. Therefore, it is envisaged that further mutations may be made in any of these regions in any CsgG monomer without affecting the ability of the monomer to form a pore that can translocate polynucleotides. It is also expected that mutations may be made in other regions, such as in any of the alpha helicies (S63 to R76, G85 to A99 or V211 to L236 of SEQ ID NO: 3) or in any of the beta sheets (1121 to N133, K135 to R142, 1146 to R162, S173 to R192 or R198 to T107 of SEQ ID NO: 3) without affecting the ability of the monomer to form a pore that can translocate polynucleotides. It is also expected that deletions of one or more amino acids can be made in any of the loop regions linking the alpha helicies and beta sheets and/or in the N-terminal and/or C-terminal regions of the CsgG monomer without affecting the ability of the monomer to form a pore that can translocate polynucleotides.

Amino acid substitutions may be made to the amino acid sequence of SEQ ID NO: 3 in addition to those discussed above, for example up to 1, 2, 3, 4, 5, 10, 20 or 30 substitutions. Conservative substitutions replace amino acids with other amino acids of similar chemical structure, similar chemical properties or similar side-chain volume. The amino acids introduced may have similar polarity, hydrophilicity, hydrophobicity, basicity, acidity, neutrality or charge to the amino acids they replace. Alternatively, the conservative substitution may introduce another amino acid that is aromatic or aliphatic in the place of a pre-existing aromatic or aliphatic amino acid. Conservative amino acid changes are well-known in the art and may be selected in accordance with the properties of the 20 main amino acids as defined in Table 1 above. Where amino acids have similar polarity, this can also be determined by reference to the hydropathy scale for amino acid side chains in Table 2.

One or more amino acid residues of the amino acid sequence of SEQ ID NO: 3 may additionally be deleted from the polypeptides described above. Up to 1, 2, 3, 4, 5, 10, 20 or 30 or more residues may be deleted.

Variants may include fragments of SEQ ID NO: 3. Such fragments retain pore forming activity. Fragments may be at least 50, at least 100, at least 150, at least 200 or at least 250 amino acids in length. Such fragments may be used to produce the pores. A fragment preferably comprises the membrane spanning domain of SEQ ID NO: 3, namely K135-Q153 and S183-S208.

One or more amino acids may be alternatively or additionally added to the polypeptides described above. An extension may be provided at the amino terminal or carboxy terminal of the amino acid sequence of SEQ ID NO: 3 or polypeptide variant or fragment thereof. The extension may be quite short, for example from 1 to 10 amino acids in length. Alternatively, the extension may be longer, for example up to 50 or 100 amino acids. A carrier protein may be fused to an amino acid sequence according to the invention. Other fusion proteins are discussed in more detail below.

A CsgG pore as described herein includes a wild type CsgG pore, or a homologue or a mutant/variant thereof. A variant is a polypeptide that has an amino acid sequence which varies from that of SEQ ID NO: 3 and which retains its ability to form a pore. A variant typically contains the regions of SEQ ID NO: 3 that are responsible for pore formation. The pore forming ability of CsgG, which contains a R-barrel, is provided by R-sheets in each subunit. A variant of SEQ ID NO: 3 typically comprises the regions in SEQ ID NO: 3 that form R-sheets, namely K134-Q154 and S183-S208. One or more modifications can be made to the regions of SEQ ID NO: 3 that form R-sheets as long as the resulting variant retains its ability to form a pore. A variant of SEQ ID NO: 3 preferably includes one or more modifications, such as substitutions, additions or deletions, within its α-helices and/or loop regions.

The mutant CsgG monomers may be a mutant CsgG monomer, which is a monomer whose sequence varies from that of a wild-type CsgG monomer and which retains the ability to form a pore. A mutant monomer may also be referred to herein as a variant. Methods for confirming the ability of mutant monomers to form pores are well-known in the art and are discussed in more detail below.

Particular pore-forming CsgG mutant monomers that may be included in the CsgG pore may comprise any one or more of the following modifications:

-   -   a W at a position corresponding to R97 in SEQ ID NO:3;     -   a W at a position corresponding to R93 in SEQ ID NO:3;     -   a Y at a position corresponding to R97 in SEQ ID NO: 3;     -   a Y at a position corresponding to R93 in SEQ ID NO: 3;     -   a Y at each of the positions corresponding to R93 and R97 in SEQ         ID NO: 3;     -   a D at the position corresponding to R192 in SEQ ID NO:3;     -   deletion of the residues at the positions corresponding to         V105-I107 in SEQ ID NO:3;     -   deletion of the residues at one or more of the positions         corresponding to F193 to L199 in SEQ ID NO: 3;     -   deletion of the residues the positions corresponding to F195 to         L199 in SEQ ID NO: 3;     -   deletion of the residues the positions corresponding to F193 to         L199 in SEQ ID NO: 3;     -   a T at the position corresponding to F191 in SEQ ID NO: 3;     -   a Q at the position corresponding to K49 in SEQ ID NO: 3;     -   a N at the position corresponding to K49 in SEQ ID NO: 3;     -   a Q at the position corresponding to K42 in SEQ ID NO: 3;     -   a Q at the position corresponding to E44 in SEQ ID NO: 3;     -   a N at the position corresponding to E44 in SEQ ID NO: 3;     -   a R at the position corresponding to L90 in SEQ ID NO: 3;     -   a R at the position corresponding to L91 in SEQ ID NO: 3;     -   a R at the position corresponding to 195 in SEQ ID NO: 3;     -   a R at the position corresponding to A99 in SEQ ID NO: 3;     -   a H at the position corresponding to E101 in SEQ ID NO: 3;     -   a K at the position corresponding to E101 in SEQ ID NO: 3;     -   a N at the position corresponding to E101 in SEQ ID NO: 3;     -   a Q at the position corresponding to E101 in SEQ ID NO: 3;     -   a T at the position corresponding to E101 in SEQ ID NO: 3;     -   a K at the position corresponding to Q114 in SEQ ID NO: 3.

The CsgG pore-forming monomer preferably further comprises an A at the position corresponding to Y51 in SEQ ID NO: 3 and/or a Q at the position corresponding to F56 in SEQ ID NO: 3.

Pores constructed from the CsgG monomers comprising a R to W substitution at the position corresponding to position 97 of SEQ ID NO: 3 display an increased accuracy as compared to otherwise identical pores without the modification at 97 when characterizing (or sequencing) target polynucleotides. An increased accuracy is also seen when instead of R97W the CsgG monomers comprise the modification R to W at a position corresponding to position 97 of SEQ ID NO: 3 or the modifications R to Y at positions corresponding to positions 93 and 97 of SEQ ID NO: 3. Accordingly, pores may be constructed from one or more mutant CsgG monomers that comprise a modification at positions corresponding to R97 or R93 of SEQ ID NO: 3 such that the modification increases the hydrophobicity of the amino acid. For example, such modification may include an amino acid substitution with any amino acid containing a hydrophobic side chain, including, e.g., but not limited to W and Y.

CsgG monomers that comprise a R to D, Q, F, S or T mutation at a position corresponding to position 192 of SEQ ID NO: 3 are easier to express than monomers which do not have a substitution at position 192 which may be due to the reduction of positive charge. Accordingly position 192 may be substituted with an amino-acid which reduces the positive charge. Monomers that comprise R192D/Q/F/S/T may also comprise additional modifications which improve the ability of mutant pores formed from the monomers to interact with and characterise analytes, such as polynucleotides. However, in one embodiment it is preferred that the residue at the position corresponding to position 193 of SEQ ID NO: 3 is R or K, more preferably R.

Pores comprising CsgG monomers that comprise a deletion of V105, A106 and I107, a deletion of F193, I194, D195, Y196, Q197, R198 and L199 or a deletion of D195, Y196, Q197, R198 and L199, and/or F191T display an increased accuracy when characterizing (or sequencing) target polynucleotides. The amino-acids at positions 105 to 107 correspond to the cis-loops in the cap of the nanopore and the amino-acids at positions 193 to 199 correspond to the trans-loops at the other end of the pore. Without wishing to be bound by theory it is thought that deletion of the cis-loops improves the interaction of the enzyme with the pore and removal of the trans-loops decreases any unwanted interaction between DNA on the trans side of the pore.

Pores comprising CsgG monomers that comprise a K to Q or K to N mutations at a position corresponding to K94 of SEQ ID NO: 3 show a reduction in the number of noisy pores (namely those pores that give rise to an increased signal:noise ratio) as compared to identical pores without the mutation at 94 when characterizing (or sequencing) target polynucleotides. Position 94 is found within the vestibule of the pore and was found to be a particularly sensitive position in relation to the noise of the current signal.

Pores comprising the CsgG monomers that comprise T104K or T104R, N91R, E101K/N/Q/T/H, E44N/Q, Q114K, A99R, I95R, N91R, L90R, E44Q/N and/or Q42K, or corresponding mutations, all demonstrate an improved ability to capture target polynucleotides when used to characterize (or sequence) target polynucleotides as compared to identical pores without substitutions at these positions.

In one embodiment, the CsgG pore comprises one or more monomers that are variants of SEQ ID NO: 3 comprising (a) one or more mutations at the following positions (i.e. mutations at one or more of the following positions) 141, R93, A98, Q100, G103, T104, A106, I107, N108, L113, S115, T117, Y130, K135, E170, S208, D233, D238 and E244 and/or (b) one or more of D43S, E44S, F48S/N/Q/Y/W/l/V/H/R/K, Q87N/R/K, N91K/R, K94R/F/Y/W/L/S/N, R97F/Y/W/V/I/K/S/Q/H, E101I/L/A/H, N102K/Q/L/l/V/S/H, R110F/G/N, Q114R/K, R142Q/S, T150Y/A/V/L/S/Q/N, R192D/Q/F/S/T and D248S/N/Q/K/R. The variant may comprise (a); (b); or (a) and (b). In some embodiments, the variant comprises R97W. In some embodiments, the variant comprises R192D/Q/F/S/T, such as R192D/Q. In (a), the variant may comprise modifications at any number and combination of the positions, such as 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18 or 19 of the positions.

In (a), the variant preferably comprises one or more of I41N, R93F/Y/W/L/I/V/N/Q/S, A98K/R, Q100K/R, G103F/W/S/N/K/R, T104R/K, A106R/K, I107R/K/W/F/Y/L/V, N108R/K, L113K/R, S115R/K, T117R/K, Y130W/F/H/Q/N, K135L/V/N/Q/S, E170S/N/Q/K/R, S208V/I/F/W/Y/L/T, D233S/N/Q/K/R, D238S/N/Q/K/R and E244S/N/Q/K/R.

In (a), the variant preferably comprises one or more modifications which provide more consistent movement of a target polynucleotide with respect to, such as through, a transmembrane pore comprising the monomer. In particular, in (a), the variant preferably comprises one or more mutations at the following positions (i.e. mutations at one or more of the following positions) R93, G103 and I107. The variant may comprise R93; G103; 1107; R93 and G103; R93 and I107; G103 and I107; or R93, G103 and I107. The variant preferably comprises one or more of R93F/Y/W/L/l/V/N/Q/S, G103F/W/S/N/K/R and I107R/K/W/F/Y/L/V. These may be present in any combination shown for the positions R93, G103 and I107.

In (a), the variant preferably comprises one or modifications which allow pores constructed from the mutant monomers preferably capture nucleotides and polynucleotides more easily. In particular, in (a), the variant preferably comprises one or more mutations at the following positions (i.e. mutations at one or more of the following positions) 141, T104, A106, N108, L113, S115, T117, E170, D233, D238 and E244. The variant may comprise modifications at any number and combination of the positions, such as 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 or 11 of the positions. The variant preferably comprises one or more of I41N, T104R/K, A106R/K, N108R/K, L113K/R, S115R/K, T117R/K, E170S/N/Q/K/R, D233S/N/Q/K/R, D238S/N/Q/K/R and E244S/N/Q/K/R. Additionally or alternatively the variant may comprise (c) Q42K/R, E44N/Q, L90R/K, N91R/K, I95R/K, A99R/K, E101H/K/N/Q/T and/or Q114K/R.

In (a), the variant preferably comprises one or more modifications which provide more consistent movement and increase capture. In particular, in (a), the variant preferably comprises one or more mutations at the following positions (i.e. mutations at one or more of the following positions) (i) A98, (ii) Q100, (iii) G103 and (iv) I107. The variant preferably comprises one or more of (i) A98R/K, (ii) Q100K/R, (iii) G103K/R and (iv) I107R/K.

Particularly preferred mutant monomers which provide for increased capture of analytes, such as a polynucleotides include a mutation at one or more of positions Q42, E44, E44, L90, N91, 195, A99, E101 and Q114, which mutation removes the negative charge and/or increases the positive charge at the mutated positions. In particular, the following mutations may be included in a mutant monomer of the invention to produce a CsgG pore that has an improved ability to capture an analyte, preferably a polynucleotide: Q42K, E44N, E44Q, L90R, N91R, I95R, A99R, E101H, E101K, E101N, E101Q, E101T and Q114K. Examples of particular mutant monomers which comprise one of these mutations in combination with other beneficial mutations are:

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-Q42K

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-E44N

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-E44Q

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-L90R

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-N91R

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-I95R

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-A99R

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-E101H

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-E101K

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-E101N

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-E101Q

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-E101T

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)-Q114K.

In (a), the variant preferably comprises one or more modifications which provide increased characterisation accuracy. In particular, in (a), the variant preferably comprises one or more mutations at the following positions (i.e. mutations at one or more of the following positions) Y130, K135 and S208, such as Y130; K135; S208; Y130 and K135; Y130 and S208; K135 and S208; or Y130, K135 and S208. The variant preferably comprises one or more of Y130W/F/H/Q/N, K135L/V/N/Q/S and R142Q/S. These substitutions may be present in any number and combination as set out for Y130, K135 and S208.

In (b), the variant may comprise any number and combination of the substitutions, such as 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 or 12 of the substitutions. In (b), the variant preferably comprises one or more modifications which provide more consistent movement of a target polynucleotide with respect to, such as through, a transmembrane pore comprising the monomer. In particular, in (b), the variant preferably comprises one or more one or more of (i) Q87N/R/K, (ii) K94R/F/Y/W/L/S/N, (iii) R97F/Y/W/V/l/K/S/Q/H, (iv) N102K/Q/L/I/V/S/H and (v) R110F/G/N. More preferably, the variant comprises K94D or K94Q and/or R97W or R97Y. Other preferred variants that are modified to provide more consistent movement of a target polynucleotide with respect to, such as through, a transmembrane pore comprising the monomer include (vi) R93W and R93Y. A preferred variant may comprise R93W and R97W, R93Y and R97W, R93W and R97W, or more preferably R93Y and R97Y.

In (b), the variant preferably comprises one or modifications which allow pores constructed from the mutant monomers preferably capture nucleotides and polynucleotides more easily. In particular, in (b), the variant preferably comprises one or more of (i) D43S, (ii) E44S, (iii) N91K/R, (iv) Q114R/K and (v) D248S/N/Q/K/R.

In (b), the variant preferably comprises one or more modifications which provide more consistent movement and increase capture. In particular, in (b), the variant preferably comprises one or more of Q87R/K, E101I/L/NH and N102K, such as Q87R/K; E101I/L/A/H; N102K; Q87R/K and E101I/L/A/H; Q87R/K and N102K; E101I/L/A/H and N102K; or Q87R/K, E101I/L/A/H and N102K.

In (b), the variant preferably comprises one or more modifications which provide increased characterisation accuracy. In particular, in (a), the variant preferably comprises F48S/N/Q/Y/W/I/V.

In (b), the variant preferably comprises one or more modifications which provide increased characterisation accuracy and increased capture. In particular, in (a), the variant preferably comprises F48H/R/K.

The variant may comprise modifications in both (a) and (b) which provide more consistent movement. The variant may comprise modifications in both (a) and (b) which provide increased capture.

The invention provides variants of SEQ ID NO: 3 which provide an increased throughput of an assay for characterising an analyte, such as a polynucleotide, using a pore comprising the variant. Such variants may comprise a mutation at K94, preferably K94Q or K94N, more preferably K94Q. Examples of particular mutant monomers which comprise a K94Q or K94N mutation in combination with other beneficial mutations are:

CsgG-(WT-Y51A/F56Q/R97W/R192D-StrepII)9-K94N

CsgG-(WT-Y51A/F56Q/R97W/R192D-StrepII)9-K94Q.

Using monomers that are variants of SEQ ID NO: 3 to form the CsgG pore can provide increased characterisation accuracy in an assay for characterising an analyte, such as a polynucleotide. Such variants include variants that comprise: a mutation at F191, preferably F191T; deletion of V105-I107; deletion of F193-L199 or of D195-L199; and/or a mutation at R93 and/or R97, preferably R93Y, R97Y, or more preferably, R97W, R93W or both R97Y and R97Y. Examples of particular mutant monomers which comprise one or more of these mutations in combination with other beneficial mutations are:

CsgG-(WT-Y51A/F56Q/R97W/R192D-StrepII)9-del(D195-L199)

CsgG-(WT-Y51A/F56Q/R97W/R192D-StrepII)9-del(F193-L199)

CsgG-(WT-Y51A/F56Q/R97W/R192D-StrepII)9-F191T

CsgG-(WT-Y51A/F56Q/R97W/R192D-del(V105-I107)-StrepII)9

CsgG-(WT-Y51A/F56Q/K94Q/R97W/R192D-del(V105-I107)

CsgG-(WT-Y51A/F56Q/R192D-StrepII)9-R93W

CsgG-(WT-Y51A/F56Q/R192D-StrepII)9-R93W-del(D195-L199)

CsgG-(WT-Y51A/F56Q/R192D-StrepII)9-R93Y/R97Y.

In another embodiment, the variant of SEQ ID NO: 3 comprises (A) deletion of one or more positions R192, F193, I194, D195, Y196, Q197, R198, L199, L200 and E201 and/or (B) deletion of one or more of V139/G140/D149/T150/V186/Q187/V204/G205 (called band 1 herein), G137/G138/Q151/Y152/Y184/E185/Y206/T207 (called band 2 herein) and A141/R142/G147/A148/A188/G189/G202/E203 (called band 3 herein).

In (A), the variant may comprise deletion of any number and combination of the positions, such as 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 of the positions. In (A), the variant preferably comprises deletion of

-   -   D195, Y196, Q197, R198 and L199;     -   R192, F193, I194, D195, Y196, Q197, R198, L199 and L200;     -   Q197, R198, L199 and L200;     -   1194, D195, Y196, Q197, R198 and L199;     -   D195, Y196, Q197, R198, L199 and L200;     -   Y196, Q197, R198, L199, L200 and E201;     -   Q197, R198, L199, L200 and E201;     -   Q197, R198, L199; or     -   F193, I194, D195, Y196, Q197, R198 and L199.

More preferably, the variant comprises deletion of D195, Y196, Q197, R198 and L199 or F193, I194, D195, Y196, Q197, R198 and L199. In (B), any number and combination of bands 1 to 3 may be deleted, such as band 1; band 2; band 3; bands 1 and 2; bands 1 and 3; bands 2 and 3; or bands 1, 2 and 3. The variant may comprise deletions according to (A); (B); or (A) and (B).

The variants comprising deletion of one or more positions according to (A) and/or (B) above may further comprise any of the modifications or substitutions discussed above and below. If the modifications or substitutions are made at one or more positions which appear after the deletion positions in SEQ ID NO: 3, the numbering of the one or more positions of the modifications or substitutions must be adjusted accordingly. For instance, if L199 is deleted, E244 becomes E243. Similarly, if band 1 is deleted, R192 becomes R186.

In another embodiment, the variant of SEQ ID NO: 3 comprises (C) deletion of one or more positions V105, A106 and I107. The deletions in accordance with (C) may be made in addition to deletions according to (A) and/or (B).

The above-described deletions typically reduce the noise associated with the movement of the target polynucleotide with respect to, such as through, a transmembrane pore comprising the monomer. As a result the target polynucleotide can be characterised more accurately.

In the paragraphs above where different amino acids at a specific positon are separated by the / symbol, the / symbol means “or”. For instance, Q87R/K means Q87R or Q87K.

The variants of SEQ ID NO: 3 which provide increased capture of an an analyte, such as a polynucleotide may comprise a mutation at T104, preferably T104R or T104K, a mutation at N91, preferably N91R, a mutation at E101, preferably E101K/N/Q/T/H, a mutation at position E44, preferably E44N or E44Q and/or a mutation at position Q42, preferably Q42K.

The mutations at different positions in SEQ ID NO: 3 may be combined in any possible way. In particular, a monomer in the CsgG pore may comprise one or more mutation that improves accuracy, one ore more mutation that reduces noise and/ore one or more mutation that enhances capture of an analyte.

The variant of SEQ ID NO: 3 preferably comprises one or more of the following (i) one or more mutations at the following positions (i.e. mutations at one or more of the following positions) N40, D43, E44, S54, S57, Q62, R97, E101, E124, E131, R142, T150 and R192, such as one or more mutations at the following positions (i.e. mutations at one or more of the following positions) N40, D43, E44, S54, S57, Q62, E101, E131 and T150 or N40, D43, E44, E101 and E131; (ii) mutations at Y51/N55, Y51/F56, N55/F56 or Y51/N55/F56; (iii) Q42R or Q42K; (iv) K49R; (v) N102R, N102F, N102Y or N102W; (vi) D149N, D149Q or D149R; (vii) E185N, E185Q or E185R; (viii) D195N, D195Q or D195R; (ix) E201N, E201Q or E201R; (x) E203N, E203Q or E203R; and (xi) deletion of one or more of the following positions F48, K49, P50, Y51, P52, A53, S54, N55, F56 and S57. The variant may comprise any combination of (i) to (xi).

If the variant comprises any one of (i) and (iii) to (xi), it may further comprise a mutation at one or more of Y51, N55 and F56, such as at Y51, N55, F56, Y51/N55, Y51/F56, N55/F56 or Y51/N55/F56.

In (i), the variant may comprises mutations at any number and combination of N40, D43, E44, S54, S57, Q62, R97, E101, E124, E131, R142, T150 and R192. In (i), the variant preferably comprises one or more mutations at the following positions (i.e. mutations at one or more of the following positions) N40, D43, E44, S54, S57, Q62, E101, E131 and T150. In (i), the variant preferably comprises one or more mutations at the following positions (i.e. mutations at one or more of the following positions) N40, D43, E44, E101 and E131. In (i), the variant preferably comprises a mutation at S54 and/or S57. In (i), the variant more preferably comprises a mutation at (a) S54 and/or S57 and (b) one or more of Y51, N55 and F56, such as at Y51, N55, F56, Y51/N55, Y51/F56, N55/F56 or Y51/N55/F56. If S54 and/or S57 are deleted in (xi), it/they cannot be mutated in (i) and vice versa. In (i), the variant preferably comprises a mutation at T150, such as T150I. Alternatively the variant preferably comprises a mutation at (a) T150 and (b) one or more of Y51, N55 and F56, such as at Y51, N55, F56, Y51/N55, Y51/F56, N55/F56 or Y51/N55/F56. In (i), the variant preferably comprises a mutation at Q62, such as Q62R or Q62K. Alternatively the variant preferably comprises a mutation at (a) Q62 and (b) one or more of Y51, N55 and F56, such as at Y51, N55, F56, Y51/N55, Y51/F56, N55/F56 or Y51/N55/F56. The variant may comprise a mutation at D43, E44, Q62 or any combination thereof, such as D43, E44, Q62, D43/E44, D43/Q62, E44/Q62 or D43/E44/Q62. Alternatively the variant preferably comprises a mutation at (a) D43, E44, Q62, D43/E44, D43/Q62, E44/Q62 or D43/E44/Q62 and (b) one or more of Y51, N55 and F56, such as at Y51, N55, F56, Y51/N55, Y51/F56, N55/F56 or Y51/N55/F56.

In (ii) and elsewhere in this application where different positions are separated by the / symbol, the / symbol means “and” such that Y51/N55 is Y51 and N55. In (ii), the variant preferably comprises mutations at Y51/N55. It has been proposed that the constriction in CsgG is composed of three stacked concentric rings formed by the side chains of residues Y51, N55 and F56 (Goyal et al, 2014, Nature, 516, 250-253). Mutation of these residues in (ii) may therefore decrease the number of nucleotides contributing to the current as the polynucleotide moves through the pore and thereby make it easier to identify a direct relationship between the observed current (as the polynucleotide moves through the pore) and the polynucleotide. F56 may be mutated in any of the ways discussed below with reference to variants and pores useful in the method of the invention.

In (v), the variant may comprise N102R, N102F, N102Y or N102W. The variant preferably comprises (a) N102R, N102F, N102Y or N102W and (b) a mutation at one or more of Y51, N55 and F56, such as at Y51, N55, F56, Y51/N55, Y51/F56, N55/F56 or Y51/N55/F56.

In (xi), any number and combination of K49, P50, Y51, P52, A53, S54, N55, F56 and S57 may be deleted. Preferably one or more of K49, P50, Y51, P52, A53, S54, N55 and S57 may be deleted. If any of Y51, N55 and F56 are deleted in (xi), it/they cannot be mutated in (ii) and vice versa.

In (i), the variant preferably comprises one of more of the following substitutions N40R, N40K, D43N, D43Q, D43R, D43K, E44N, E44Q, E44R, E44K, S54P, S57P, Q62R, Q62K, R97N, R97G, R97L, E101N, E101Q, E101R, E101K, E101F, E101Y, E101W, E124N, E124Q, E124R, E124K, E124F, E124Y, E124W, E131D, R142E, R142N, T150I, R192E and R192N, such as one or more of N40R, N40K, D43N, D43Q, D43R, D43K, E44N, E44Q, E44R, E44K, S54P, S57P, Q62R, Q62K, E101N, E101Q, E101R, E101K, E101F, E101Y, E101W, E131 D and T150I, or one or more of N40R, N40K, D43N, D43Q, D43R, D43K, E44N, E44Q, E44R, E44K, E101N, E101Q, E101R, E101K, E101F, E101Y, E101W and E131 D. The variant may comprise any number and combination of these substitutions. In (i), the variant preferably comprises S54P and/or S57P. In (i), the variant preferably comprises (a) S54P and/or S57P and (b) a mutation at one or more of Y51, N55 and F56, such as at Y51, N55, F56, Y51/N55, Y51/F56, N55/F56 or Y51/N55/F56. The mutations at one or more of Y51, N55 and F56 may be any of those discussed below. In (i), the variant preferably comprises F56A/S57P or S54P/F56A. The variant preferably comprises T150I. Alternatively the variant preferably comprises a mutation at (a) T150I and (b) one or more of Y51, N55 and F56, such as at Y51, N55, F56, Y51/N55, Y51/F56, N55/F56 or Y51/N55/F56.

In (i), the variant preferably comprises Q62R or Q62K. Alternatively the variant preferably comprises (a) Q62R or Q62K and (b) a mutation at one or more of Y51, N55 and F56, such as at Y51, N55, F56, Y51/N55, Y51/F56, N55/F56 or Y51/N55/F56. The variant may comprise D43N, E44N, Q62R or Q62K or any combination thereof, such as D43N, E44N, Q62R, Q62K, D43N/E44N, D43N/Q62R, D43N/Q62K, E44N/Q62R, E44N/Q62K, D43N/E44N/Q62R or D43N/E44N/Q62K. Alternatively the variant preferably comprises (a) D43N, E44N, Q62R, Q62K, D43N/E44N, D43N/Q62R, D43N/Q62K, E44N/Q62R, E44N/Q62K, D43N/E44N/Q62R or D43N/E44N/Q62K and (b) a mutation at one or more of Y51, N55 and F56, such as at Y51, N55, F56, Y51/N55, Y51/F56, N55/F56 or Y51/N55/F56.

In (i), the variant preferably comprises D43N.

In (i), the variant preferably comprises E101R, E101 S, E101F or E101N.

In (i), the variant preferably comprises E124N, E124Q, E124R, E124K, E124F, E124Y, E124W or E124D, such as E124N.

In (i), the variant preferably comprises R142E and R142N.

In (i), the variant preferably comprises R97N, R97G or R97L.

In (i), the variant preferably comprises R192E and R192N.

In (ii), the variant preferably comprises F56N/N55Q, F56N/N55R, F56N/N55K, F56N/N55S, F56N/N55G, F56N/N55A, F56N/N55T, F56Q/N55Q, F56Q/N55R, F56Q/N55K, F56Q/N55S, F56Q/N55G, F56Q/N55A, F56Q/N55T, F56R/N55Q, F56R/N55R, F56R/N55K, F56R/N55S, F56R/N55G, F56R/N55A, F56R/N55T, F56S/N55Q, F56S/N55R, F56S/N55K, F56S/N55S, F56S/N55G, F56S/N55A, F56S/N55T, F56G/N55Q, F56G/N55R, F56G/N55K, F56G/N55S, F56G/N55G, F56G/N55A, F56G/N55T, F56A/N55Q, F56A/N55R, F56A/N55K, F56A/N55S, F56A/N55G, F56A/N55A, F56A/N55T, F56K/N55Q, F56K/N55R, F56K/N55K, F56K/N55S, F56K/N55G, F56K/N55A, F56K/N55T, F56N/Y51L, F56N/Y51V, F56N/Y51A, F56N/Y51N, F56N/Y51Q, F56N/Y51S, F56N/Y51G, F56Q/Y51L, F56Q/Y51V, F56Q/Y51A, F56Q/Y51N, F56Q/Y51Q, F56Q/Y51S, F56Q/Y51G, F56R/Y51L, F56R/Y51V, F56R/Y51A, F56R/Y51N, F56R/Y51Q, F56R/Y51S, F56R/Y51G, F56S/Y51L, F56S/Y51V, F56S/Y51A, F56S/Y51N, F56S/Y51Q, F56S/Y51S, F56S/Y51G, F56G/Y51L, F56G/Y51V, F56G/Y51A, F56G/Y51N, F56G/Y51Q, F56G/Y51S, F56G/Y51G, F56A/Y51L, F56A/Y51V, F56A/Y51A, F56A/Y51N, F56A/Y51Q, F56A/Y51S, F56A/Y51G, F56K/Y51L, F56K/Y51V, F56K/Y51A, F56K/Y51N, F56K/Y51Q, F56K/Y51S, F56K/Y51G, N55Q/Y51L, N55Q/Y51V, N55Q/Y51A, N55Q/Y51N, N55Q/Y51Q, N55Q/Y51S, N55Q/Y51G, N55R/Y51L, N55R/Y51V, N55R/Y51A, N55R/Y51N, N55R/Y51Q, N55R/Y51S, N55R/Y51G, N55K/Y51L, N55K/Y51V, N55K/Y51A, N55K/Y51N, N55K/Y51Q, N55K/Y51S, N55K/Y51G, N55S/Y51L, N55S/Y51V, N55S/Y51A, N55S/Y51N, N55S/Y51Q, N55S/Y51S, N55S/Y51G, N55G/Y51L, N55G/Y51V, N55G/Y51A, N55G/Y51N, N55G/Y51Q, N55G/Y51S, N55G/Y51G, N55A/Y51L, N55A/Y51V, N55A/Y51A, N55A/Y51N, N55A/Y51Q, N55A/Y51S, N55A/Y51G, N55T/Y51L, N55T/Y51V, N55T/Y51A, N55T/Y51N, N55T/Y51Q, N55T/Y51S, N55T/Y51G, F56N/N55Q/Y51L, F56N/N55Q/Y51V, F56N/N55Q/Y51A, F56N/N55Q/Y51N, F56N/N55Q/Y51Q, F56N/N55Q/Y51S, F56N/N55Q/Y51G, F56N/N55R/Y51L, F56N/N55R/Y51V, F56N/N55R/Y51A, F56N/N55R/Y51N, F56N/N55R/Y51Q, F56N/N55R/Y51S, F56N/N55R/Y51G, F56N/N55K/Y51L, F56N/N55K/Y51V, F56N/N55K/Y51A, F56N/N55K/Y51N, F56N/N55K/Y51Q, F56N/N55K/Y51S, F56N/N55K/Y51G, F56N/N55S/Y51L, F56N/N55S/Y51V, F56N/N55S/Y51A, F56N/N55S/Y51N, F56N/N55S/Y51Q, F56N/N55S/Y51S, F56N/N55S/Y51G, F56N/N55G/Y51L, F56N/N55G/Y51V, F56N/N55G/Y51A, F56N/N55G/Y51N, F56N/N55G/Y51Q, F56N/N55G/Y51S, F56N/N55G/Y51G, F56N/N55A/Y51L, F56N/N55A/Y51V, F56N/N55A/Y51A, F56N/N55A/Y51N, F56N/N55A/Y51Q, F56N/N55A/Y51S, F56N/N55A/Y51G, F56N/N55T/Y51L, F56N/N55T/Y51V, F56N/N55T/Y51A, F56N/N55T/Y51N, F56N/N55T/Y51Q, F56N/N55T/Y51S, F56N/N55T/Y51G, F56Q/N55Q/Y51L, F56Q/N55Q/Y51V, F56Q/N55Q/Y51A, F56Q/N55Q/Y51N, F56Q/N55Q/Y51Q, F56Q/N55Q/Y51S, F56Q/N55Q/Y51G, F56Q/N55R/Y51L, F56Q/N55R/Y51V, F56Q/N55R/Y51A, F56Q/N55R/Y51N, F56Q/N55R/Y51Q, F56Q/N55R/Y51S, F56Q/N55R/Y51G, F56Q/N55K/Y51L, F56Q/N55K/Y51V, F56Q/N55K/Y51A, F56Q/N55K/Y51N, F56Q/N55K/Y51Q, F56Q/N55K/Y51S, F56Q/N55K/Y51G, F56Q/N55S/Y51L, F56Q/N55S/Y51V, F56Q/N55S/Y51A, F56Q/N55S/Y51N, F56Q/N55S/Y51Q, F56Q/N55S/Y51S, F56Q/N55S/Y51G, F56Q/N55G/Y51L, F56Q/N55G/Y51V, F56Q/N55G/Y51A, F56Q/N55G/Y51N, F56Q/N55G/Y51Q, F56Q/N55G/Y51S, F56Q/N55G/Y51G, F56Q/N55A/Y51L, F56Q/N55A/Y51V, F56Q/N55A/Y51A, F56Q/N55A/Y51N, F56Q/N55A/Y51Q, F56Q/N55A/Y51S, F56Q/N55A/Y51G, F56Q/N55T/Y51L, F56Q/N55T/Y51V, F56Q/N55T/Y51A, F56Q/N55T/Y51N, F56Q/N55T/Y51Q, F56Q/N55T/Y51S, F56Q/N55T/Y51G, F56R/N55Q/Y51L, F56R/N55Q/Y51V, F56R/N55Q/Y51A, F56R/N55Q/Y51N, F56R/N55Q/Y51Q, F56R/N55Q/Y51S, F56R/N55Q/Y51G, F56R/N55R/Y51L, F56R/N55R/Y51V, F56R/N55R/Y51A, F56R/N55R/Y51N, F56R/N55R/Y51Q, F56R/N55R/Y51S, F56R/N55R/Y51G, F56R/N55K/Y51L, F56R/N55K/Y51V, F56R/N55K/Y51A, F56R/N55K/Y51N, F56R/N55K/Y51Q, F56R/N55K/Y51S, F56R/N55K/Y51G, F56R/N55S/Y51L, F56R/N55S/Y51V, F56R/N55S/Y51A, F56R/N55S/Y51N, F56R/N55S/Y51Q, F56R/N55S/Y51S, F56R/N55S/Y51G, F56R/N55G/Y51L, F56R/N55G/Y51V, F56R/N55G/Y51A, F56R/N55G/Y51N, F56R/N55G/Y51Q, F56R/N55G/Y51S, F56R/N55G/Y51G, F56R/N55A/Y51L, F56R/N55A/Y51V, F56R/N55A/Y51A, F56R/N55A/Y51N, F56R/N55A/Y51Q, F56R/N55A/Y51S, F56R/N55A/Y51G, F56R/N55T/Y51L, F56R/N55T/Y51V, F56R/N55T/Y51A, F56R/N55T/Y51N, F56R/N55T/Y51Q, F56R/N55T/Y51S, F56R/N55T/Y51G, F56S/N55Q/Y51L, F56S/N55Q/Y51V, F56S/N55Q/Y51A, F56S/N55Q/Y51N, F56S/N55Q/Y51Q, F56S/N55Q/Y51S, F56S/N55Q/Y51G, F56S/N55R/Y51L, F56S/N55R/Y51V, F56S/N55R/Y51A, F56S/N55R/Y51N, F56S/N55R/Y51Q, F56S/N55R/Y51S, F56S/N55R/Y51G, F56S/N55K/Y51L, F56S/N55K/Y51V, F56S/N55K/Y51A, F56S/N55K/Y51N, F56S/N55K/Y51Q, F56S/N55K/Y51S, F56S/N55K/Y51G, F56S/N55S/Y51L, F56S/N55S/Y51V, F56S/N55S/Y51A, F56S/N55S/Y51N, F56S/N55S/Y51Q, F56S/N55S/Y51S, F56S/N55S/Y51G, F56S/N55G/Y51L, F56S/N55G/Y51V, F56S/N55G/Y51A, F56S/N55G/Y51N, F56S/N55G/Y51Q, F56S/N55G/Y51S, F56S/N55G/Y51G, F56S/N55A/Y51L, F56S/N55A/Y51V, F56S/N55A/Y51A, F56S/N55A/Y51N, F56S/N55A/Y51Q, F56S/N55A/Y51S, F56S/N55A/Y51G, F56S/N55T/Y51L, F56S/N55T/Y51V, F56S/N55T/Y51A, F56S/N55T/Y51N, F56S/N55T/Y51Q, F56S/N55T/Y51S, F56S/N55T/Y51G, F56G/N55Q/Y51L, F56G/N55Q/Y51V, F56G/N55Q/Y51A, F56G/N55Q/Y51N, F56G/N55Q/Y51Q, F56G/N55Q/Y51S, F56G/N55Q/Y51G, F56G/N55R/Y51L, F56G/N55R/Y51V, F56G/N55R/Y51A, F56G/N55R/Y51N, F56G/N55R/Y51Q, F56G/N55R/Y51S, F56G/N55R/Y51G, F56G/N55K/Y51L, F56G/N55K/Y51V, F56G/N55K/Y51A, F56G/N55K/Y51N, F56G/N55K/Y51Q, F56G/N55K/Y51S, F56G/N55K/Y51G, F56G/N55S/Y51L, F56G/N55S/Y51V, F56G/N55S/Y51A, F56G/N55S/Y51N, F56G/N55S/Y51Q, F56G/N55S/Y51S, F56G/N55S/Y51G, F56G/N55G/Y51L, F56G/N55G/Y51V, F56G/N55G/Y51A, F56G/N55G/Y51N, F56G/N55G/Y51Q, F56G/N55G/Y51S, F56G/N55G/Y51G, F56G/N55A/Y51L, F56G/N55A/Y51V, F56G/N55A/Y51A, F56G/N55A/Y51N, F56G/N55A/Y51Q, F56G/N55A/Y51S, F56G/N55A/Y51G, F56G/N55T/Y51L, F56G/N55T/Y51V, F56G/N55T/Y51A, F56G/N55T/Y51N, F56G/N55T/Y51Q, F56G/N55T/Y51S, F56G/N55T/Y51G, F56A/N55Q/Y51L, F56A/N55Q/Y51V, F56A/N55Q/Y51A, F56A/N55Q/Y51N, F56A/N55Q/Y51Q, F56A/N55Q/Y51S, F56A/N55Q/Y51G, F56A/N55R/Y51L, F56A/N55R/Y51V, F56A/N55R/Y51A, F56A/N55R/Y51N, F56A/N55R/Y51Q, F56A/N55R/Y51S, F56A/N55R/Y51G, F56A/N55K/Y51L, F56A/N55K/Y51V, F56A/N55K/Y51A, F56A/N55K/Y51N, F56A/N55K/Y51Q, F56A/N55K/Y51S, F56A/N55K/Y51G, F56A/N55S/Y51L, F56A/N55S/Y51V, F56A/N55S/Y51A, F56A/N55S/Y51N, F56A/N55S/Y51Q, F56A/N55S/Y51S, F56A/N55S/Y51G, F56A/N55G/Y51L, F56A/N55G/Y51V, F56A/N55G/Y51A, F56A/N55G/Y51N, F56A/N55G/Y51Q, F56A/N55G/Y51S, F56A/N55G/Y51G, F56A/N55A/Y51L, F56A/N55A/Y51V, F56A/N55A/Y51A, F56A/N55A/Y51N, F56A/N55A/Y51Q, F56A/N55A/Y51S, F56A/N55A/Y51G, F56A/N55T/Y51L, F56A/N55T/Y51V, F56A/N55T/Y51A, F56A/N55T/Y51N, F56A/N55T/Y51Q, F56A/N55T/Y51S, F56A/N55T/Y51G, F56K/N55Q/Y51L, F56K/N55Q/Y51V, F56K/N55Q/Y51A, F56K/N55Q/Y51N, F56K/N55Q/Y51Q, F56K/N55Q/Y51S, F56K/N55Q/Y51G, F56K/N55R/Y51L, F56K/N55R/Y51V, F56K/N55R/Y51A, F56K/N55R/Y51N, F56K/N55R/Y51Q, F56K/N55R/Y51S, F56K/N55R/Y51G, F56K/N55K/Y51L, F56K/N55K/Y51V, F56K/N55K/Y51A, F56K/N55K/Y51N, F56K/N55K/Y51Q, F56K/N55K/Y51S, F56K/N55K/Y51G, F56K/N55S/Y51L, F56K/N55S/Y51V, F56K/N55S/Y51A, F56K/N55S/Y51N, F56K/N55S/Y51Q, F56K/N55S/Y51S, F56K/N55S/Y51G, F56K/N55G/Y51L, F56K/N55G/Y51V, F56K/N55G/Y51A, F56K/N55G/Y51N, F56K/N55G/Y51Q, F56K/N55G/Y51S, F56K/N55G/Y51G, F56K/N55A/Y51L, F56K/N55A/Y51V, F56K/N55A/Y51A, F56K/N55A/Y51N, F56K/N55A/Y51Q, F56K/N55A/Y51S, F56K/N55A/Y51G, F56K/N55T/Y51L, F56K/N55T/Y51V, F56K/N55T/Y51A, F56K/N55T/Y51N, F56K/N55T/Y51Q, F56K/N55T/Y51S, F56K/N55T/Y51G, F56E/N55R, F56E/N55K, F56D/N55R, F56D/N55K, F56R/N55E, F56R/N55D, F56K/N55E or F56K/N55D.

In (ii), the variant preferably comprises Y51R/F56Q, Y51N/F56N, Y51M/F56Q, Y51L/F56Q, Y51I/F56Q, Y51V/F56Q, Y51A/F56Q, Y51P/F56Q, Y51G/F56Q, Y51C/F56Q, Y51Q/F56Q, Y51N/F56Q, Y51S/F56Q, Y51E/F56Q, Y51D/F56Q, Y51K/F56Q or Y51H/F56Q.

In (ii), the variant preferably comprises Y51T/F56Q, Y51Q/F56Q or Y51A/F56Q.

In (ii), the variant preferably comprises Y51T/F56F, Y51T/F56M, Y51T/F56L, Y51T/F561, Y51T/F56V, Y51T/F56A, Y51T/F56P, Y51T/F56G, Y51T/F56C, Y51T/F56Q, Y51T/F56N, Y51T/F56T, Y51T/F56S, Y51T/F56E, Y51T/F56D, Y51T/F56K, Y51T/F56H or Y51T/F56R.

In (ii), the variant preferably comprises Y51T/N55Q, Y51T/N55S or Y51T/N55A.

In (ii), the variant preferably comprises Y51A/F56F, Y51A/F56L, Y51A/F561, Y51A/F56V, Y51A/F56A, Y51A/F56P, Y51A/F56G, Y51A/F56C, Y51A/F56Q, Y51A/F56N, Y51A/F56T, Y51A/F56S, Y51A/F56E, Y51A/F56D, Y51A/F56K, Y51A/F56H or Y51A/F56R.

In (ii), the variant preferably comprises Y51C/F56A, Y51E/F56A, Y51D/F56A, Y51K/F56A, Y51H/F56A, Y51Q/F56A, Y51N/F56A, Y51S/F56A, Y51P/F56A or Y51V/F56A.

In (xi), the variant preferably comprises deletion of Y51/P52, Y51/P52/A53, P50 to P52, P50 to A53, K49 to Y51, K49 to A53 and replacement with a single proline (P), K49 to S54 and replacement with a single P, Y51 to A53, Y51 to S54, N55/F56, N55 to S57, N55/F56 and replacement with a single P, N55/F56 and replacement with a single glycine (G), N55/F56 and replacement with a single alanine (A), N55/F56 and replacement with a single P and Y51N, N55/F56 and replacement with a single P and Y51Q, N55/F56 and replacement with a single P and Y51S, N55/F56 and replacement with a single G and Y51N, N55/F56 and replacement with a single G and Y51Q, N55/F56 and replacement with a single G and Y51S, N55/F56 and replacement with a single A and Y51N, N55/F56 and replacement with a single A/Y51Q or N55/F56 and replacement with a single A and Y51S.

The variant more preferably comprises D195N/E203N, D195Q/E203N, D195N/E203Q, D195Q/E203Q, E201N/E203N, E201Q/E203N, E201N/E203Q, E201Q/E203Q, E185N/E203Q, E185Q/E203Q, E185N/E203N, E185Q/E203N, D195N/E201N/E203N, D195Q/E201N/E203N, D195N/E201Q/E203N, D195N/E201N/E203Q, D195Q/E201Q/E203N, D195Q/E201N/E203Q, D195N/E201Q/E203Q, D195Q/E201Q/E203Q, D149N/E201N, D149Q/E201N, D149N/E201Q, D149Q/E201Q, D149N/E201N/D195N, D149Q/E201N/D195N, D149N/E201Q/D195N, D149N/E201N/D195Q, D149Q/E201Q/D195N, D149Q/E201N/D195Q, D149N/E201Q/D195Q, D149Q/E201Q/D195Q, D149N/E203N, D149Q/E203N, D149N/E203Q, D149Q/E203Q, D149N/E185N/E201N, D149Q/E185N/E201N, D149N/E185Q/E201N, D149N/E185N/E201Q, D149Q/E185Q/E201N, D149Q/E185N/E201Q, D149N/E185Q/E201Q, D149Q/E185Q/E201Q, D149N/E185N/E203N, D149Q/E185N/E203N, D149N/E185Q/E203N, D149N/E185N/E203Q, D149Q/E185Q/E203N, D149Q/E185N/E203Q, D149N/E185Q/E203Q, D149Q/E185Q/E203Q, D149N/E185N/E201N/E203N, D149Q/E185N/E201N/E203N, D149N/E185Q/E201N/E203N, D149N/E185N/E201Q/E203N, D149N/E185N/E201N/E203Q, D149Q/E185Q/E201N/E203N, D149Q/E185N/E201Q/E203N, D149Q/E185N/E201N/E203Q, D149N/E185Q/E201Q/E203N, D149N/E185Q/E201N/E203Q, D149N/E185N/E201Q/E203Q, D149Q/E185Q/E201Q/E203Q, D149Q/E185Q/E201N/E203Q, D149Q/E185N/E201Q/E203Q, D149N/E185Q/E201Q/E203Q, D149Q/E185Q/E201Q/E203N, D149N/E185N/D195N/E201N/E203N, D149Q/E185N/D195N/E201N/E203N, D149N/E185Q/D195N/E201N/E203N, D149N/E185N/D195Q/E201N/E203N, D149N/E185N/D195N/E201Q/E203N, D149N/E185N/D195N/E201N/E203Q, D149Q/E185Q/D195N/E201N/E203N, D149Q/E185N/D195Q/E201N/E203N, D149Q/E185N/D195N/E201Q/E203N, D149Q/E185N/D195N/E201N/E203Q, D149N/E185Q/D195Q/E201N/E203N, D149N/E185Q/D195N/E201Q/E203N, D149N/E185Q/D195N/E201N/E203Q, D149N/E185N/D195Q/E201Q/E203N, D149N/E185N/D195Q/E201N/E203Q, D149N/E185N/D195N/E201Q/E203Q, D149Q/E185Q/D195Q/E201N/E203N, D149Q/E185Q/D195N/E201Q/E203N, D149Q/E185Q/D195N/E201N/E203Q, D149Q/E185N/D195Q/E201Q/E203N, D149Q/E185N/D195Q/E201N/E203Q, D149Q/E185N/D195N/E201Q/E203Q, D149N/E185Q/D195Q/E201Q/E203N, D149N/E185Q/D195Q/E201N/E203Q, D149N/E185Q/D195N/E201Q/E203Q, D149N/E185N/D195Q/E201Q/E203Q, D149Q/E185Q/D195Q/E201Q/E203N, D149Q/E185Q/D195Q/E201N/E203Q, D149Q/E185Q/D195N/E201Q/E203Q, D149Q/E185N/D195Q/E201Q/E203Q, D149N/E185Q/D195Q/E201Q/E203Q, D149Q/E185Q/D195Q/E201Q/E203Q, D149N/E185R/E201N/E203N, D149Q/E185R/E201N/E203N, D149N/E185R/E201Q/E203N, D149N/E185R/E201N/E203Q, D149Q/E185R/E201Q/E203N, D149Q/E185R/E201N/E203Q, D149N/E185R/E201Q/E203Q, D149Q/E185R/E201Q/E203Q, D149R/E185N/E201N/E203N, D149R/E185Q/E201N/E203N, D149R/E185N/E201Q/E203N, D149R/E185N/E201N/E203Q, D149R/E185Q/E201Q/E203N, D149R/E185Q/E201N/E203Q, D149R/E185N/E201Q/E203Q, D149R/E185Q/E201Q/E203Q, D149R/E185N/D195N/E201N/E203N, D149R/E185Q/D195N/E201N/E203N, D149R/E185N/D195Q/E201N/E203N, D149R/E185N/D195N/E201Q/E203N, D149R/E185Q/D195N/E201N/E203Q, D149R/E185Q/D195Q/E201N/E203N, D149R/E185Q/D195N/E201Q/E203N, D149R/E185Q/D195N/E201N/E203Q, D149R/E185N/D195Q/E201Q/E203N, D149R/E185N/D195Q/E201N/E203Q, D149R/E185N/D195N/E201Q/E203Q, D149R/E185Q/D195Q/E201Q/E203N, D149R/E185Q/D195Q/E201N/E203Q, D149R/E185Q/D195N/E201Q/E203Q, D149R/E185N/D195Q/E201Q/E203Q, D149R/E185Q/D195Q/E201Q/E203Q, D149N/E185R/D195N/E201N/E203N, D149Q/E185R/D195N/E201N/E203N, D149N/E185R/D195Q/E201N/E203N, D149N/E185R/D195N/E201Q/E203N, D149N/E185R/D195N/E201N/E203Q, D149Q/E185R/D195Q/E201N/E203N, D149Q/E185R/D195N/E201Q/E203N, D149Q/E185R/D195N/E201N/E203Q, D149N/E185R/D195Q/E201Q/E203N, D149N/E185R/D195Q/E201N/E203Q, D149N/E185R/D195N/E201Q/E203Q, D149Q/E185R/D195Q/E201Q/E203N, D149Q/E185R/D195Q/E201N/E203Q, D149Q/E185R/D195N/E201Q/E203Q, D149N/E185R/D195Q/E201Q/E203Q, D149Q/E185R/D195Q/E201Q/E203Q, D149N/E185R/D195N/E201R/E203N, D149Q/E185R/D195N/E201R/E203N, D149N/E185R/D195Q/E201R/E203N, D149N/E185R/D195N/E201R/E203Q, D149Q/E185R/D195Q/E201R/E203N, D149Q/E185R/D195N/E201R/E203Q, D149N/E185R/D195Q/E201R/E203Q, D149Q/E185R/D195Q/E201R/E203Q, E131D/K49R, E101N/N102F, E101N/N102Y, E101N/N102W, E101F/N102F, E101F/N102Y, E101F/N102W, E101Y/N102F, E101Y/N102Y, E101Y/N102W, E101W/N102F, E101W/N102Y, E101W/N102W, E101N/N102R, E101F/N102R, E101Y/N102R or E101W/N102F.

Preferred variants of the invention which form pores in which fewer nucleotides contribute to the current as the polynucleotide moves through the pore comprise Y51A/F56A, Y51A/F56N, Y51I/F56A, Y51L/F56A, Y51T/F56A, Y51I/F56N, Y51L/F56N or Y51T/F56N or more preferably Y51I/F56A, Y51L/F56 Å or Y51T/F56A. As discussed above, this makes it easier to identify a direct relationship between the observed current (as the polynucleotide moves through the pore) and the polynucleotide.

Preferred variants which form pores displaying an increased range comprise mutations at the following positions:

Y51, F56, D149, E185, E201 and E203;

N55 and F56;

Y51 and F56;

Y51, N55 and F56; or

F56 and N102.

Preferred variants which form pores displaying an increased range comprise:

Y51N, F56A, D149N, E185R, E201N and E203N;

N55S and F56Q;

Y51A and F56A;

Y51A and F56N;

Y51I and F56A;

Y51L and F56A;

Y51T and F56A;

Y51I and F56N;

Y51L and F56N;

Y51T and F56N;

Y51T and F56Q;

Y51A, N55S and F56A;

Y51A, N55S and F56N;

Y51T, N55S and F56Q; or

F56Q and N102R.

Preferred variants which form pores in which fewer nucleotides contribute to the current as the polynucleotide moves through the pore comprise mutations at the following positions:

N55 and F56, such as N55X and F56Q, wherein X is any amino acid; or

Y51 and F56, such as Y51X and F56Q, wherein X is any amino acid.

Particularly preferred variants comprise Y51A and F56Q.

Preferred variants which form pores displaying an increased throughput comprise mutations at the following positions:

D149, E185 and E203;

D149, E185, E201 and E203; or

D149, E185, D195, E201 and E203.

Preferred variants which form pores displaying an increased throughput comprise:

D149N, E185N and E203N;

D149N, E185N, E201N and E203N;

D149N, E185R, D195N, E201N and E203N; or

D149N, E185R, D195N, E201R and E203N.

Preferred variants which form pores in which capture of the polynucleotide is increased comprise the following mutations:

D43N/Y51T/F56Q;

E44N/Y51T/F56Q;

D43N/E44N/Y51T/F56Q;

Y51T/F56Q/Q62R;

D43N/Y51T/F56Q/Q62R;

E44N/Y51T/F56Q/Q62R; or

D43N/E44N/Y51T/F56Q/Q62R.

Preferred variants comprise the following mutations:

D149R/E185R/E201R/E203R or Y51T/F56Q/D149R/E185R/E201R/E203R;

D149N/E185N/E201N/E203N or Y51T/F56Q/D149N/E185N/E201N/E203N;

E201R/E203R or Y51T/F56Q/E201R/E203R

E201N/E203R or Y51T/F56Q/E201N/E203R;

E203R or Y51T/F56Q/E203R;

E203N or Y51T/F56Q/E203N;

E201R or Y51T/F56Q/E201R;

E201N or Y51T/F56Q/E201N;

E185R or Y51T/F56Q/E185R;

E185N or Y51T/F56Q/E185N;

D149R or Y51T/F56Q/D149R;

D149N or Y51T/F56Q/D149N;

R142E or Y51T/F56Q/R142E;

R142N or Y51T/F56Q/R142N;

R192E or Y51T/F56Q/R192E; or

R192N or Y51T/F56Q/R192N.

Preferred variants comprise the following mutations:

Y51A/F56Q/E101N/N102R;

Y51A/F56Q/R97N/N102G;

Y51A/F56Q/R97N/N102R;

Y51A/F56Q/R97N;

Y51A/F56Q/R97G;

Y51A/F56Q/R97L;

Y51A/F56Q/N102R;

Y51A/F56Q/N102F;

Y51A/F56Q/N102G;

Y51A/F56Q/E101R;

Y51A/F56Q/E101F;

Y51A/F56Q/E101N; or

Y51A/F56Q/E101 G

The variant preferably further comprises a mutation at T150. A preferred variant which forms a pore displaying an increased insertion comprises T150I. A mutation at T150, such as T150I, may be combined with any of the mutations or combinations of mutations discussed above.

A preferred variant of SEQ ID NO: 3 comprises (a) R97W and (b) a mutation at Y51 and/or F56. A preferred variant of SEQ ID NO: 3 comprises (a) R97W and (b) Y51R/H/K/D/E/S/T/N/Q/C/G/P/A/V/I/L/M and/or F56 R/H/K/D/E/S/T/N/Q/C/G/P/A/V/I/L/M. A preferred variant of SEQ ID NO: 3 comprises (a) R97W and (b) Y51L/A/N/Q/S/G and/or F56A/Q/N. A preferred variant of SEQ ID NO: 3 comprises (a) R97W and (b) Y51A and/or F56Q. A preferred variant of SEQ ID NO: 3 comprises R97W, Y51A and F56Q.

The variant of SEQ ID NO: 3 preferably comprises a mutation at R192. The variant preferably comprises R192D/Q/F/S/T/N/E, R192D/Q/F/S/T or R192D/Q. A preferred variant of SEQ ID NO: 3 comprises (a) R97W, (b) a mutation at Y51 and/or F56 and (c) a mutation at R192, such as R192D/Q/F/S/T/N/E, R192D/Q/F/S/T or R192D/Q. A preferred variant of SEQ ID NO: 3 comprises (a) R97W, (b) Y51R/H/K/D/E/S/T/N/Q/C/G/P/A/V/I/L/M and/or F56 R/H/K/D/E/S/T/N/Q/C/G/P/A/V/I/L/M and (c) a mutation at R192, such as R192D/Q/F/S/T/N/E, R192D/Q/F/S/T or R192D/Q. A preferred variant of SEQ ID NO: 3 comprises (a) R97W, (b) Y51LV/A/N/Q/S/G and/or F56A/Q/N and (c) a mutation at R192, such as R192D/Q/F/S/T/N/E, R192D/Q/F/S/T or R192D/Q. A preferred variant of SEQ ID NO: 3 comprises (a) R97W, (b) Y51A and/or F56Q and (c) a mutation at R192, such as R192 D/Q/F/S/T/N/E, R192D/Q/F/S/T or R192D/Q. A preferred variant of SEQ ID NO: 3 comprises R97W, Y51A, F56Q and R192D/Q/F/S/T or R192D/Q. A preferred variant of SEQ ID NO: 3 comprises R97W, Y51A, F56Q and R192D. A preferred variant of SEQ ID NO: 3 comprises R97W, Y51A, F56Q and R192Q. In the paragraphs above where different amino acids at a specific positon are separated by the / symbol, the / symbol means “or”. For instance, R192D/Q means R192D or R192Q.

Any of the above preferred variants of SEQ ID NO: 3 described above may further comprises a mutation at R93. A preferred variant of SEQ ID NO: 3 comprise (a) R93W and (b) a mutation at Y51 and/or F56, preferably Y51A and F56Q.

Any of the above preferred variants of SEQ ID NO: 3 may comprise a K94N/Q mutation. Any of the above preferred variants of SEQ ID NO: 3 may comprise a F191T mutation.

The CsgG monomer may be modified to facilitate attachment to the CsgF peptide. For example a cysteine residue may be introduced at one or more of the positions corresponding to positions 132, 133, 136, 138, 140, 142, 144, 145, 147, 149, 151, 153, 155, 183, 185, 187, 189, 191, 201, 203, 205, 207 and 209 of SEQ ID NO: 3, and/or at any one of the positions identified in Table 4 as being predicted to make contact with CsgF, to facilitate covalent attachment to CsgG. As an alternative or addition to covalent attachment via cysteine residues, the pore may be stabilised by hydrophobic interactions or electrostatic interactions. To facilitate such interactions, a non-native reactive or photoreactive amino acid at a position corresponding to one or more of positions 132, 133, 136, 138, 140, 142, 144, 145, 147, 149, 151, 153, 155, 183, 185, 187, 189, 191, 201, 203, 205, 207 and 209 of SEQ ID NO: 3, and/or at any one of the positions identified in Table 4 as being predicted to make contact with CsgF.

Preferred exemplary pores include at least one CsgG monomer having the following mutations relative to SEQ ID NO: 3: Y51X₁/N55X₂/F56X₃/N91R/K94Q/R97W/R192D-del(V105-I107), wherein X₁ is I/V/S/T, X₂ is N/I/V/S/T and/or X₃ is Q/I/V/S/T.

Methods for introducing or substituting naturally-occurring amino acids are well known in the art. For instance, methionine (M) may be substituted with arginine (R) by replacing the codon for methionine (ATG) with a codon for arginine (CGT) at the relevant position in a polynucleotide encoding the mutant monomer. The polynucleotide can then be expressed as discussed below.

Double Pores

The CsgG/CsgF pore may be a double pore comprising a first pore and a second pore. At least the first pore is a CsgG/CsgF pore disclosed herein. The second pore may be a CsgG pore or a CsgG/CsgF pore. In one embodiment both the first pore and the second pore are CsgG/CsgF pores as disclosed herein. The first and second pores may be the same or different. In addition to any of the mutations disclosed herein, in a double pore, the CsgG monomer may comprise one or more of the additional mutations described below.

In the double pore, the first pore may be attached to the second CsgG pore by hydrophobic interactions and/or by one or more disulphide bond. One or more, such as 2, 3, 4, 5, 6, 8, 9, for example all, of the monomers in the first pore and/or the second pore may be modified to enhance such interactions. This may be achieved in any suitable way.

At least one cysteine residue in the amino acid sequence of the first pore at the interface between the first and second pores may be disulphide bonded to at least one cysteine residue in the amino acid sequence of the second pore at the interface between the first and second pores. The cysteine residue in the first pore and/or the cysteine residue in the second pore may be a cysteine residue that is not present in the wild type CsgG monomer. Multiple disulphide bonds, such as from 2, 3, 4, 5, 6, 7, 8 or 9 to 16, 18, 24, 27, 32, 36, 40, 45, 48, 54, 56 or 63, may form between the two pores in the double pore. One or both the first or second pore may comprise at least one monomer, such as up to 8, 9 or 10 monomers, that comprises a cysteine residue at the interface between the first and second pores at a position corresponding to R97, I107, R110, Q100, E101, N102 and/or L113 of SEQ ID NO: 3.

At least one monomer in the first pore and/or at least one monomer in the second pore may comprise at least one residue at the interface between the first and second pores, which residue is more hydrophobic than the residue present at the corresponding position in the wild type CsgG monomer. For example, from 2 to 10, such as 3, 4, 5, 6, 7, 8 or 9, residues in the first pore and/or the second pore may be more hydrophobic that the residues at the same positions in the corresponding wild type CsgG monomer. Such hydrophobic residues strengthen the interaction between the two pores in the double pore. The at least one residue at the interface between the first and second pores may be at a position corresponding to R97, I107, R110, Q100, E101, N102 and or L113 of SEQ ID NO: 3. Where the residue at the interface in the wild type CsgG monomer is R, Q, N or E, the hydrophobic residue is typically I, L, V, M, F, W or Y. Where the residue at the interface in the wild type CsgG monomer is I, the hydrophobic residue is typically L, V, M, F, W or Y. Where the residue at the interface in the wild type CsgG monomer is L, the hydrophobic residue is typically I, V, M, F, W or Y.

The double pore may comprise one or more monomer that comprises one or more cysteine residue at the interface between the pores and one or more monomer that comprises one or more introduced hydrophobic residue at the interface between the pores, or may comprise one or more monomer that comprises such cysteine residues and such hydrophobic residues. For example, one or more, such as any 2, 3, or 4, of the positions in the monomer corresponding to the positions at R97, I107, R110, Q100, E101, N102 and or L113 of SEQ ID NO: 3 may comprise a cysteine (C) residue and one or more, such as any 2, 3 or 4, of the positions in the monomer corresponding to the positions at R97, I107, R110, Q100, E101, N102 and or L113 of SEQ ID NO: 3 may comprise a hydrophobic residue, such as I, L, V, M, F, W or Y.

The double pore according may contain bulky residues at one or more, such as 2, 3, 4, 5, 6 or 7, positions in the tail region, which residues are typically at the interface between the first and second pores and are bulkier than the residues present at the corresponding positions in the wild type CsgG monomer. The bulk of these residues prevents holes from forming in the walls of the pore at the interface between the first and second pore in the double pore. The at least one bulky residue at the interface between the first and second pores is typically at a position corresponding to A98, A99, T104, V105, L113, Q114 or S115 of SEQ ID NO: 3. Where the residue at the interface in the wild type CsgG monomer is A, the bulky residue is typically I, L, V, M, F, W, Y, N, Q, S or T. Where the residue present at the interface in the wild type CsgG monomer is T, the bulky residue is typically L, M, F, W, Y, N, Q, R, D or E. Where the residue present at the interface in the wild type CsgG monomer is V, the bulky residue is typically I, L, M, F, W, Y, N, Q. Where the residue present at the interface in the wild type CsgG monomer is L, the bulky residue is typically M, F, W, Y, N, Q, R, D or E. Where the residue present at the interface in the wild type CsgG monomer is Q, the bulky residue is typically F, W or Y. Where the residue present at the interface in the wild type CsgG monomer is S, the bulky residue is typically M, F, W, Y, N, Q, E or R.

Particularly where the second pore is located outside the membrane, the second pore, and optionally the first pore, preferably comprises residues in the barrel region of the pore that reduce the negative charge inside the barrel compared to the charge in the barrel of the wild type CsgG pore. These mutations make the barrel more hydrophilic. At least one monomer in the first pore and/or at least one monomer in the second pore of the double pore may comprise at least one residue in the barrel region of the pore, which residue has less negative charge than the residue present at the corresponding position in the wild type CsgG monomer. The charge inside the barrel is sufficiently neutral or positive such that negatively charged analytes, such as polynucleotides, are not repelled from entering the pore by electrostatic charges. At least one residue, such as 2, 3, 4 or 5 residues, in the barrel region of the pore at a position corresponding to D149, E185, D195, E210 and/or E203 of SEQ ID NO: 3 may be a neutral or positively charged amino acid. At least one residue, such as 2, 3, 4 or 5 residues, in the barrel region of the pore at a position corresponding to D149, E185, D195, E210 and/or E203 of SEQ ID NO: 3 is preferably N, Q, R or K.

Particular examples of charge-removing mutations in SEQ ID NO: 3 include the following: E185N/E203N; D149N/E185R/D195N/E201R/E203N, D149N/E185R/D195N/E201N/E203N, D149R/E185N/D195N/E201N/E203N, D149R/E185N/E201N/E203N, D149N/E185N/D195/E201N/E203N, D149N/E185N/E201N/E203N, D149N/E185N/E203N, D149N/E185N/E201N, D149N/E203N, D149N/E201N/D195N, D149N/E201N, D195N/E201N/E203N, E201N/E203N, D195N/E203, E203R, E203N, E201R, E201N, D195R, D195N, E185R, E185N, D149R and D149N.

At least one CsgG monomer in the first pore may comprise at least one residue in the constriction of the barrel region of the first pore, which residue decreases, maintains or increases the length of the constriction compared to the wild type CsgG pore and/or at least one monomer in the second CsgG pore may comprise at least one residue in the constriction of the barrel region of the second pore, which residue decreases, maintains or increases the length of the constriction compared to the wild type CsgG pore. Preferably, the length of the constriction in the first pore and/or the length of the constriction in the second pore is at least as long as in the wild-type pore and more preferably longer.

The length of the pore may be increased by inserting residues into the region corresponding to the region between positions K49 and F56 of SEQ ID NO: 3. From 1 to 5, such as 2, 3, or 4 amino acid residues may be inserted at any one or more of the following positions defined by reference to SEQ ID NO: 3: K49 and P50, P50 and Y51, Y51 and P52, P52 and A53, A53 and S54, S54 and N55 and/or N55 and F56. Preferably from 1 to 10, such as 2 to 8, or 3 to 5 amino acid residues in total are inserted into the sequence of a monomer. Preferably, all of the monomers in the first pore and/or all of the monomers in the second pore have the same number of insertions in this region. The inserted residues may increase the length of the loop between the residues corresponding to Y51 and N55 of SEQ ID NO: 3. The inserted residues may be any combination of A, S, G or T to maintain flexibility; P to add a kink to the loop; and/or S, T, N, Q, M, F, W, Y, V and/or I to contribute to the signal produced when a analyte interacts with the barrel of the pore under an applied potential difference. The inserted amino acids may be any combination of S, G, SG, SGG, SGS, GS, GSS and/or GSG.

In the double pore, the constriction in the barrel of the first pore and/or the second pore may comprise at least one residue, such as 2, 3, 4 or 5 residues, which influences the properties of the pore when used to detect or characterise an analyte compared to when a first pore or a second pore with a wild-type constriction is used, wherein the at least one residue in the constriction of the barrel region of the pore is at a position corresponding to Y51, N55, Y51, P52 and/or A53 of SEQ ID NO: 3. The at least one residue may be Q or V at a position corresponding to F56 of SEQ ID NO: 3; A or Q at a position corresponding to Y51 of SEQ ID NO: 3; and/or V at a position corresponding to N55 of SEQ ID NO: 3.

The double pore may comprise at least one monomer in the first CsgG pore and/or at least one monomer in the second CsgG pore, which monomer comprises two or more of the mutations defined above.

A CsgG monomer in the double pore may comprise a cysteine residue at a position corresponding to R97, I107, R110, Q100, E101, N102 and or L113 of SEQ ID NO: 3.

A CsgG monomer in the double pore may comprise a residue at a position corresponding to any one or more of R97, Q100, I107, R110, E101, N102 and L113 of SEQ ID NO: 3, which residue is more hydrophobic than the residue present at the corresponding position of SEQ ID NO: 3, such as the corresponding position of any one of SEQ ID NOs: 68 to 88, wherein the residue at the position corresponding to R97 and/or I107 is M, the residue at the position corresponding to R110 is I, L, V, M, W or Y, and/or the residue at the position corresponding to E101 or N102 is V or M. The residue at a position corresponding to Q100 is typically I, L, V, M, F, W or Y; and or the residue at a position corresponding to L113 is typically I, V, M, F, W or Y.

Particular monomers may have the sequence shown in SEQ ID NO 3 comprising Y51A, F56Q substitutions and R97I/V/L/M/F/W/Y, I107L/V/M/F/W/Y, R110I/V/L/M/F/W/Y, Q100I/V/L/M/F/W/Y, E101I/V/L/M/F/W/Y, N102I/V/L/M/F/W/Y and L113CI/V/L/M/F/W/Y in combination, R97I/V/L/M/F/W/Y and N102I/V/L/M/F/W/Y in combination and/or R97I/V/L/M/F/W/Y and E101I/V/L/M/F/W/Y in combination. I107 may already form hydrophobic interactions between two pores.

The CsgG monomer in at least one of the pores in the double pore may comprise a residue at a position corresponding to any one or more of A98, A99, T104, V105, L113, Q114 and S115 of SEQ ID NO: 3 which is bulkier than the residue present at the corresponding position of SEQ ID NO: 3, such as the corresponding position of any one of SEQ ID NOs: 68 to 88, wherein the residue at the position corresponding to T104 is L, M, F, W, Y, N, Q, D or E, the residue at the position corresponding to L113 is M, F, W, Y, N, G, D or E and/or the residue at the position corresponding to S115 is M, F, W, Y, N, Q or E. The residue at a position corresponding to A98 or A99, is typically I, L, V, M, F, W, Y, N, Q, S or T. The residue at a position corresponding to V105 is I, L, M, F, W, Y, N or Q. The residue at a position corresponding to Q114 is F, W or Y. The residue at a position corresponding to E210 is N, Q, R or K.

Particular monomers may have the sequence shown in SEQ ID NO 3 comprising Y51A, F56Q substitutions and 1, 2, 3, 4, 5, 6 or all of the following substitutions: A981/L/V/M/F/W/Y/N/Q/S/T; A991/L/V/M/F/W/Y/N/Q/S/T; T104N/Q/L/R/D/E/M/F/W/Y; V105I/L/M/F/W/Y/N/Q; L113M/F/W/Y/N/Q/D/E/L/R; Q114Y/F/W; and S115N/Q/M/F/W/Y/E/R.

The CsgG monomer in at least one of the pores in the double pore may comprise a residue in the barrel region of the pore at a position corresponding to any one ore more of D149, E185, D195, E210 and E203 less negative charge than the residue present at the corresponding position of SEQ ID NO: 3, such as the corresponding position of any one of SEQ ID NOs: 68 to 88, wherein the residue at the position corresponding to D149, E185, D195 and/or E203 is K.

The CsgG monomer in at least one of the pores in the double pore may comprise at least one residue in the constriction of the barrel region of the pore, which residue increases the length of the constriction compared to the wild type CsgG pore. The at least one residue is additional to the residues present in the constriction of the wild type CsgG pore.

The length of the pore may be increased by inserting residues into the region corresponding to the region between positions K49 and F56 of SEQ ID NO: 3. From 1 to 5, such as 2, 3, or 4 amino acid residues may be inserted at any one or more of the following positions defined by reference to SEQ ID NO: 3: K49 and P50, P50 and Y51, Y51 and P52, P52 and A53, A53 and S54, S54 and N55 and/or N55 and F56. Preferably from 1 to 10, such as 2 to 8, or 3 to 5 amino acid residues in total are inserted into the sequence of the monomer. The inserted residues may increase the length of the loop between the residues corresponding to Y51 and N55 of SEQ ID NO: 3. The inserted residues may be any combination of A, S, G or T to maintain flexibility; P to add a kink to the loop; and/or S, T, N, Q, M, F, W, Y, V and/or I to contribute to the signal produced when a analyte interacts with the barrel of the pore under an applied potential difference. The inserted amino acids may be any combination of S, G, SG, SGG, SGS, GS, GSS and/or GSG.

The CsgG monomer in at least one of the pores in the double pore may comprise at least one residue in the constriction of the barrel region of the pore at a position corresponding to N55, P52 and/or A53 of SEQ ID NO: 3 that is different from the residue present in the corresponding wild type monomer, wherein the residue at a position corresponding to N55 is V.

Any two or more of the above described residues may be present in the same monomer. In particular the monomer may comprise at least one said cysteine residue, at least one said hydrophobic residue, at least one said bulky residue, at least one said neutral or positively charged residue and/or at least one said residue that increases the length of the constriction.

A CsgG monomer in the double pore may additionally comprise one or more, such as 2, 3, 4 or 5 residues, which influence the properties of the pore when used to detect or characterise an analyte compared to when a first pore or a second pore with a wild-type constriction is used, wherein the at least one residue in the constriction of the barrel region of the pore is at a position corresponding to Y51, N55, Y51, P52 and/or A53 of SEQ ID NO: 3. The at least one residue may be Q or V at a position corresponding to F56 of SEQ ID NO: 3; A or Q at a position corresponding to Y51 of SEQ ID NO: 3; and/or V at a position corresponding to N55 of SEQ ID NO: 3.

Method for Making Modified Proteins

Methods for introducing or substituting non-naturally-occurring amino acids are also well known in the art. For instance, non-naturally-occurring amino acids may be introduced by including synthetic aminoacyl-tRNAs in the IVTT system used to express the mutant monomer. Alternatively, they may be introduced by expressing the mutant monomer in E. coli that are auxotrophic for specific amino acids in the presence of synthetic (i.e. non-naturally-occurring) analogues of those specific amino acids. They may also be produced by naked ligation if the mutant monomer is produced using partial peptide synthesis.

The monomers derived from CsgG may be modified to assist their identification or purification, for example by the addition of a streptavidin tag or by the addition of a signal sequence to promote their secretion from a cell where the monomer does not naturally contain such a sequence. Other suitable tags are discussed in more detail below. The monomer may be labelled with a revealing label. The revealing label may be any suitable label which allows the monomer to be detected. Suitable labels are described below.

The monomer derived from CsgG may also be produced using D-amino acids. For instance, the monomer derived from CsgG may comprise a mixture of L-amino acids and D-amino acids. This is conventional in the art for producing such proteins or peptides.

The monomer derived from CsgG contains one or more specific modifications to facilitate nucleotide discrimination. The monomer derived from CsgG may also contain other non-specific modifications as long as they do not interfere with pore formation. A number of non-specific side chain modifications are known in the art and may be made to the side chains of the monomer derived from CsgG. Such modifications include, for example, reductive alkylation of amino acids by reaction with an aldehyde followed by reduction with NaBH₄, amidination with methylacetimidate or acylation with acetic anhydride.

The monomer derived from CsgG can be produced using standard methods known in the art. The monomer derived from CsgG may be made synthetically or by recombinant means. For example, the monomer may be synthesised by in vitro translation and transcription (IVTT). Suitable methods for producing pores and monomers are discussed in the International applications WO 2010/004273, WO 2010/004265 or WO 2010/086603. Methods for inserting pores into membranes are known.

Two or more CsgG monomers in the pore may be covalently attached to one another. For example, at least 2, at least 3, at least 4, at least 5, at least 6, at least 7, at least 8, at least 9 or at least 10 monomers may be covalently attached. The covalently attached monomers may be the same or different.

The monomers may be genetically fused, optionally via a linker, or chemically fused, for instance via a chemical crosslinker. Methods for covalently attaching monomers are disclosed in WO2017/149316, WO2017/149317 and WO2017/149318.

In some embodiments, the mutant monomer is chemically modified. The mutant monomer can be chemically modified in any way and at any site. The mutant monomer is preferably chemically modified by attachment of a molecule to one or more cysteines (cysteine linkage), attachment of a molecule to one or more lysines, attachment of a molecule to one or more non-natural amino acids, enzyme modification of an epitope or modification of a terminus. Suitable methods for carrying out such modifications are well-known in the art. The mutant monomer may be chemically modified by the attachment of any molecule. For instance, the mutant monomer may be chemically modified by attachment of a dye or a fluorophore.

In some embodiments, the mutant monomer is chemically modified with a molecular adaptor that facilitates the interaction between a pore comprising the monomer and a target nucleotide or target polynucleotide sequence. The presence of the adaptor improves the host-guest chemistry of the pore and the nucleotide or polynucleotide sequence and thereby improves the sequencing ability of pores formed from the mutant monomer. The principles of host-guest chemistry are well-known in the art. The adaptor has an effect on the physical or chemical properties of the pore that improves its interaction with the nucleotide or polynucleotide sequence. The adaptor may alter the charge of the barrel or channel of the pore or specifically interact with or bind to the nucleotide or polynucleotide sequence thereby facilitating its interaction with the pore.

The molecular adaptor is preferably a cyclic molecule, a cyclodextrin, a species that is capable of hybridization, a DNA binder or interchelator, a peptide or peptide analogue, a synthetic polymer, an aromatic planar molecule, a small positively-charged molecule or a small molecule capable of hydrogen-bonding.

The adaptor may be cyclic. A cyclic adaptor preferably has the same symmetry as the pore. The adaptor preferably has eight-fold or nine-fold symmetry since CsgG typically has eight or nine subunits around a central axis. This is discussed in more detail below.

The adaptor typically interacts with the nucleotide or polynucleotide sequence via host-guest chemistry. The adaptor is typically capable of interacting with the nucleotide or polynucleotide sequence. The adaptor comprises one or more chemical groups that are capable of interacting with the nucleotide or polynucleotide sequence. The one or more chemical groups preferably interact with the nucleotide or polynucleotide sequence by non-covalent interactions, such as hydrophobic interactions, hydrogen bonding, Van der Waal's forces, π-cation interactions and/or electrostatic forces. The one or more chemical groups that are capable of interacting with the nucleotide or polynucleotide sequence are preferably positively charged. The one or more chemical groups that are capable of interacting with the nucleotide or polynucleotide sequence more preferably comprise amino groups. The amino groups can be attached to primary, secondary or tertiary carbon atoms. The adaptor even more preferably comprises a ring of amino groups, such as a ring of 6, 7 or 8 amino groups. The adaptor most preferably comprises a ring of eight amino groups. A ring of protonated amino groups may interact with negatively charged phosphate groups in the nucleotide or polynucleotide sequence.

The correct positioning of the adaptor within the pore can be facilitated by host-guest chemistry between the adaptor and the pore comprising the mutant monomer. The adaptor preferably comprises one or more chemical groups that are capable of interacting with one or more amino acids in the pore. The adaptor more preferably comprises one or more chemical groups that are capable of interacting with one or more amino acids in the pore via non-covalent interactions, such as hydrophobic interactions, hydrogen bonding, Van der Waal's forces, π-cation interactions and/or electrostatic forces. The chemical groups that are capable of interacting with one or more amino acids in the pore are typically hydroxyls or amines. The hydroxyl groups can be attached to primary, secondary or tertiary carbon atoms. The hydroxyl groups may form hydrogen bonds with uncharged amino acids in the pore. Any adaptor that facilitates the interaction between the pore and the nucleotide or polynucleotide sequence can be used.

Suitable adaptors include, but are not limited to, cyclodextrins, cyclic peptides and cucurbiturils. The adaptor is preferably a cyclodextrin or a derivative thereof. The cyclodextrin or derivative thereof may be any of those disclosed in Eliseev, A. V., and Schneider, H-J. (1994) J. Am. Chem. Soc. 116, 6081-6088. The adaptor is more preferably heptakis-6-amino-β-cyclodextrin (am₇-βCD), 6-monodeoxy-6-monoamino-β-cyclodextrin (am₁-□CD) or heptakis-(6-deoxy-6-guanidino)-cyclodextrin (gu₇-βCD). The guanidino group in gu₇-βCD has a much higher pKa than the primary amines in am₇-βCD and so it is more positively charged. This gu₇-βCD adaptor may be used to increase the dwell time of the nucleotide in the pore, to increase the accuracy of the residual current measured, as well as to increase the base detection rate at high temperatures or low data acquisition rates.

If a succinimidyl 3-(2-pyridyldithio)propionate (SPDP) crosslinker is used as discussed in more detail below, the adaptor is preferably heptakis(6-deoxy-6-amino)-6-N-mono(2-pyridyl)dithiopropanoyl-β-cyclodextrin (am₆amPDP₁-βCD).

More suitable adaptors include γ-cyclodextrins, which comprise 9 sugar units (and therefore have nine-fold symmetry). The γ-cyclodextrin may contain a linker molecule or may be modified to comprise all or more of the modified sugar units used in the β-cyclodextrin examples discussed above.

The molecular adaptor may be covalently attached to the mutant monomer. The adaptor can be covalently attached to the pore using any method known in the art. The adaptor is typically attached via chemical linkage. If the molecular adaptor is attached via cysteine linkage, the one or more cysteines have preferably been introduced to the mutant, for instance in the barrel, by substitution. The mutant monomer may be chemically modified by attachment of a molecular adaptor to one or more cysteines in the mutant monomer. The one or more cysteines may be naturally-occurring, i.e. at positions 1 and/or 215 in SEQ ID NO: 3. Alternatively, the mutant monomer may be chemically modified by attachment of a molecule to one or more cysteines introduced at other positions. The cysteine at position 215 may be removed, for instance by substitution, to ensure that the molecular adaptor does not attach to that position rather than the cysteine at position 1 or a cysteine introduced at another position.

The reactivity of cysteine residues may be enhanced by modification of the adjacent residues. For instance, the basic groups of flanking arginine, histidine or lysine residues will change the pKa of the cysteines thiol group to that of the more reactive S⁻ group. The reactivity of cysteine residues may be protected by thiol protective groups such as dTNB. These may be reacted with one or more cysteine residues of the mutant monomer before a linker is attached.

The molecule may be attached directly to the mutant monomer. The molecule is preferably attached to the mutant monomer using a linker, such as a chemical crosslinker or a peptide linker.

Suitable chemical crosslinkers are well-known in the art. Preferred crosslinkers include 2,5-dioxopyrrolidin-1-yl 3-(pyridin-2-yldisulfanyl)propanoate, 2,5-dioxopyrrolidin-1-yl 4-(pyridin-2-yldisulfanyl)butanoate and 2,5-dioxopyrrolidin-1-yl 8-(pyridin-2-yldisulfanyl)octananoate. The most preferred crosslinker is succinimidyl 3-(2-pyridyldithio)propionate (SPDP). Typically, the molecule is covalently attached to the bifunctional crosslinker before the molecule/crosslinker complex is covalently attached to the mutant monomer but it is also possible to covalently attach the bifunctional crosslinker to the monomer before the bifunctional crosslinker/monomer complex is attached to the molecule.

The linker is preferably resistant to dithiothreitol (DTT). Suitable linkers include, but are not limited to, iodoacetamide-based and Maleimide-based linkers.

In other embodiment, the monomer may be attached to a polynucleotide binding protein. This forms a modular sequencing system that may be used in the methods of sequencing of the invention. Polynucleotide binding proteins are discussed below.

The polynucleotide binding protein is preferably covalently attached to the mutant monomer. The protein can be covalently attached to the monomer using any method known in the art. The monomer and protein may be chemically fused or genetically fused. The monomer and protein are genetically fused if the whole construct is expressed from a single polynucleotide sequence. Genetic fusion of a monomer to a polynucleotide binding protein is discussed in WO 2010/004265.

If the polynucleotide binding protein is attached via cysteine linkage, the one or more cysteines have preferably been introduced to the mutant by substitution. The one or more cysteines are preferably introduced into loop regions which have low conservation amongst homologues indicating that mutations or insertions may be tolerated. They are therefore suitable for attaching a polynucleotide binding protein. In such embodiments, the naturally-occurring cysteine at position 251 may be removed. The reactivity of cysteine residues may be enhanced by modification as described above.

The polynucleotide binding protein may be attached directly to the mutant monomer or via one or more linkers. The molecule may be attached to the mutant monomer using the hybridization linkers described in as WO 2010/086602. Alternatively, peptide linkers may be used. Peptide linkers are amino acid sequences. The length, flexibility and hydrophilicity of the peptide linker are typically designed such that it does not to disturb the functions of the monomer and molecule. Preferred flexible peptide linkers are stretches of 2 to 20, such as 4, 6, 8, 10 or 16, serine and/or glycine amino acids. More preferred flexible linkers include (SG)₁, (SG)2, (SG)₃, (SG)₄, (SG)₅ and (SG)₈ wherein S is serine and G is glycine. Preferred rigid linkers are stretches of 2 to 30, such as 4, 6, 8, 16 or 24, proline amino acids. More preferred rigid linkers include (P)₁₂ wherein P is proline.

Chemical Modification

The mutant CsgG monomer or CsgF peptide may be chemically modified with a molecular adaptor and a polynucleotide binding protein.

The molecule (with which the monomer or peptide is chemically modified) may be attached directly to the monomer or peptide or attached via a linker as disclosed in WO 2010/004273, WO 2010/004265 or WO 2010/086603.

Any of the proteins described herein, such as the CsgG monomers and/or CsgF peptides, may be modified to assist their identification or purification, for example by the addition of histidine residues (a his tag), aspartic acid residues (an asp tag), a streptavidin tag, a flag tag, a SUMO tag, a GST tag or a MBP tag, or by the addition of a signal sequence to promote their secretion from a cell where the polypeptide does not naturally contain such a sequence. An alternative to introducing a genetic tag is to chemically react a tag onto a native or engineered position on the protein. An example of this would be to react a gel-shift reagent to a cysteine engineered on the outside of the protein. This has been demonstrated as a method for separating hemolysin hetero-oligomers (Chem Biol. 1997 July; 4(7):497-505).

Any of the proteins described herein, such as the CsgG monomers and/or CsgF peptides, may be labelled with a revealing label. The revealing label may be any suitable label which allows the protein to be detected. Suitable labels include, but are not limited to, fluorescent molecules, radioisotopes, e.g. ¹²⁵I, ³⁵S, enzymes, antibodies, antigens, polynucleotides and ligands such as biotin.

Any of the proteins described herein, such as the CsgG monomers and/or CsgF peptides, may be made synthetically or by recombinant means. For example, the protein may be synthesised by in vitro translation and transcription (IVTT). The amino acid sequence of the protein may be modified to include non-naturally occurring amino acids or to increase the stability of the protein. When a protein is produced by synthetic means, such amino acids may be introduced during production. The protein may also be altered following either synthetic or recombinant production.

Proteins may also be produced using D-amino acids. For instance, the protein may comprise a mixture of L-amino acids and D-amino acids. This is conventional in the art for producing such proteins or peptides.

The protein may also contain other non-specific modifications as long as they do not interfere with the function of the protein. A number of non-specific side chain modifications are known in the art and may be made to the side chains of the protein(s). Such modifications include, for example, reductive alkylation of amino acids by reaction with an aldehyde followed by reduction with NaBH₄, amidination with methylacetimidate or acylation with acetic anhydride.

Any of the proteins described herein, such as the CsgG monomers and/or CsgF peptides, can be produced using standard methods known in the art. Polynucleotide sequences encoding a protein may be derived and replicated using standard methods in the art. Polynucleotide sequences encoding a protein may be expressed in a bacterial host cell using standard techniques in the art. The protein may be produced in a cell by in situ expression of the polypeptide from a recombinant expression vector. The expression vector optionally carries an inducible promoter to control the expression of the polypeptide. These methods are described in Sambrook, J. and Russell, D. (2001). Molecular Cloning: A Laboratory Manual, 3rd Edition. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y.

Proteins may be produced in large scale following purification by any protein liquid chromatography system from protein producing organisms or after recombinant expression. Typical protein liquid chromatography systems include FPLC, AKTA systems, the Bio-Cad system, the Bio-Rad BioLogic system and the Gilson HPLC system.

Method of Producing Pores

In a third aspect, the invention provides methods to in vivo and in vitro produce CsgG: modified CsgF pore complex holding two or more constriction sites. One embodiment provides a method for producing a transmembrane pore complex, comprising a CsgG pore, or homologue or mutant form thereof, and the modified CsgF peptide, or its homologue or mutant, via co-expression. Said method comprising the steps of expressing CsgG monomers (expressed as preprotein provided in SEQ ID NO: 2, or a homologue or mutant thereof), and expressing modified or truncated CsgF monomers, both in a suitable host cell, allowing in vivo complex pore formation. Said complex comprises modified CsgF peptides, in complex with the CsgG pore, to provide the pore with an additional reader head. The resulting pore complex produced by said method using modified CsgF peptides provides a structure that is sufficient for a use of the pore complex in characterization of target analytes such as nucleic acid sequencing, as it allows passage of the analytes, in particular polynucleotide strands, and comprises two or more reader heads for improved reading of said polynucleotide sequence, when used in the appropriate settings for said application.

More particularly, the modified CsgF peptide expressed in said method comprises the preprotein depicted in SEQ ID NO: 8, 10, 12, or 14, or homologues thereof. Those sequences limiting the method to those CsgF fragments capable of introducing a constriction site in the pore complex, and binding to the CsgG protein pore, to obtain a biological pore.

Another method for producing an isolated pore complex formed by the CsgG and CsgF proteins or the like, relates to in vitro reconstitution of said monomers to obtain a functional pore. Said method comprises the steps of contacting the mature CsgG monomer as depicted in SEQ ID NO:3 or a homologue or mutant thereof, with modified CsgF peptides, or homologues or mutants thereof, in a suitable system to allow complex formation. Said system may be an “in vitro system”, which refers to a system comprising at least the necessary components and environment to execute said method, and makes use of biological molecules, organisms, a cell (or part of a cell) outside of their normal naturally-occurring environment, permitting a more detailed, more convenient, or more efficient analysis than can be done with whole organisms. An in vitro system may also comprise a suitable buffer composition provided in a test tube, wherein said protein components to form the complex have been added. A person skilled in the art is aware of the options to provide said system. Said modified CsgF peptides or the like applied in said method for in vitro reconstitution in particular embodiments are peptides comprising SEQ ID NO:15 or SEQ ID NO:16, or mutant or homologues thereof, which can be synthesized or recombinantly produced. Alternatively, modified CsgF peptides comprising SEQ ID NO:40, 39, 38, or 37, 15, 54, 55 or homologues or mutants thereof, are provided in said method for contacting with CsgG or CsgG-like pores to produce the pore complex.

The CsgG/CsgF pore can be made by any suitable method. Examples of such suitable methods are described.

In one embodiment, the CsgG/CsgF pore may be produced by co-expression. In this embodiment, at least one gene encoding a CsgG monomer polypeptide (which may be a mutant polypeptide) in one vector and a gene encoding at least one full length or truncated CsgF polypeptide (which may be a mutant polypeptide) in a second vector may be transformed together to express the proteins and make the complex within transformed cells. This could be in vivo or in vitro. Alternatively, the two genes encoding the polypeptides of CsgG and CsgF can be placed in one vector under the control of a single promotor or under the control of two separate promoters, which may be the same or different.

In another embodiment, the CsgG/CsgF pore is produced by expressing the CsgG monomer(s) separately from the CsgF peptide. CsgG monomers or a CsgG pore may be purified from the cells transformed with a vector encoding at least one CsgG monomer, or with more than one vector each expressing a CsgG monomer. The CsgF peptide may be purified from the cells transformed with a vector encoding at least one CsgF peptide. The purified CsgG monomer(s)/pore may then be incubated together with the CsgF peptide(s) to make the pore complex.

In another embodiment, the CsgG monomer(s) and/or the CsgF peptide are produced separately by in vitro translation and transcription (IVTT). The CsgG monomer(s) may then be incubated together with the CsgF peptide(s) to make the pore complex. Use of this method is illustrated in FIG. 14.

The above embodiments may be combined, such that for example, (i) CsgG is produced in vivo and CsgF in vivo; (ii) CsgG is produced in vitro and CsgF in vivo; (iii) CsgG is produced in vivo and CsgF in vitro; or (iv) CsgG is produced in vitro and CsgF in vitro.

One or both of the CsgG monomer and the CsgF peptide may be tagged to facilitate purification. Purification can also be performed when the CsgG monomer and/or CsgF peptide are untagged. Methods known in the art (e.g. ion exchange, gel filtration, hydrophobic interaction column chromatography etc.) can be used alone or in different combinations to purify the components of the pore.

Any known tags can be used in any of the two proteins. In one embodiment, two tag purification can be used to purify the CsgG:CsgF complex from CsgG pore and CsgF. For example, a Strep tag can be used in CsgG and His tag can be used in CsgF or vice versa. This is exemplified in FIG. 13. A similar end result can be obtained when the two proteins are purified individually and mixed together followed by another round of Strep and His purification.

When the full length CsgF protein forms the complex with CsgG, the neck and head domains of CsgF (FIG. 4B) (shown in the red box) protrudes out from the beta barrel of the CsgG pore.

Therefore, if a pore comprising a CsgG pore and the full length CsgF were used in single channel recording experiments, the head domain may hinder or prevent pore insertion in to a membrane. They may also potentially block the passage of the pore for analytes to pass through. Thus, when inserting a pore into a membrane, it is better if the number of flexible polypeptides dangling down from the beta-barrel are reduced. Truncated versions of the CsgF protein that mimic the FCP region resolved in the cryo EM structure of the complex and that maintain structural integrity are provided herein.

The CsgG/CsgF pore can be made prior to insertion into a membrane or after insertion of the CsgG pore into a membrane. When the pore complex is made prior to insertion into a membrane, a truncation mutant is preferably used. However, the CsgG pore may be inserted into a membrane and the CsgF peptide may be added afterwards so that the CsgG and CsgF complex can form in situ. For example, in one embodiment a system where the trans side of the membrane is accessible (for example in a chip or chamber for electrophysiogy measurements), the CsgG pore may be inserted into the membrane, and then a CsgF peptide may be added from the trans side of the membrane, so that the complex can be formed in-situ. In any embodiment in which the CsgG pore is formed in situ, larger CsgF peptide may be used. For example, the CsgF peptide may comprise all or part of the neck domain of CsgF (from about residue 36 of SEQ ID NO: 6). In some embodiments the CsgF may comprise all of the neck domain and part of the head domain (residues 36 to XX of SEQ ID NO: 6)

Depending on the methods of making the complex and the stability of the complex with a particular truncation, CsgG:CsgF and CsgG:FCP complexes can be made in different methods.

In one embodiment, the truncated version of the CsgF polypeptide at the required length is used directly.

Another embodiment uses the full length polypeptide of CsgF or a longer than required truncation (enough to keep the complex stable) in which a protease cleavage sites is inserted (e.g. TEV, HRV 3 or any other protease cleavage site) so that cleavage by the protease produces a CsgF peptide of the required length. In this embodiment, once the CsgG/CsgF complex is formed, the protease is used to cleave the CsgF at the required site. Alternatively, the protease may be used to produce the CsgF peptide prior to complex assembly.

Some protease sites will leave an additional tag behind after cleavage. For example, the TEV protease cleavage sequence is ENLYFQS. TEV protease cleaves the protein between Q and S leaving ENLYFQ intact at the C-terminus of the CsgF peptide. FIG. 15 shows an example using a modified CsgF comprising a TEV cleavage site which is cleaved using TEV protease after complex formation. Byway of another example, the HRV C3 cleavage site is LEVLFQGP and the enzyme cleaves between Q and G leaving LEVLFQ intact at the C-terminus of the CsgF peptide.

Methods of Characterising an Analyte

In a further aspect, the invention provides a method of determining the presence, absence or one or more characteristics of a target analyte. The method involves contacting the target analyte with an isolated pore complex, or transmembrane pore, such as a pore of the invention, such that the target analyte moves with respect to, such as into or through, the pore channel and taking one or more measurements as the analyte moves with respect to the pore and thereby determining the presence, absence or one or more characteristics of the analyte. The target analyte may also be called the template analyte or the analyte of interest. The isolated pore complex typically comprises at least 7, at least 8, at least 9 or at least 10 monomers, such as 7, 8, 9 or 10 CsgG monomers. The isolated pore complex preferably comprises eight or nine identical CsgG monomers. One or more, such as 2, 3, 4, 5, 6, 7, 8, 9 or 10, of the CsgG monomers is preferably chemically modified, or the CsgF peptide is chemically modified. The isolated pore complex monomers, such as the CsgG monomers, or homologues or mutants thereof, and the modified CsgF monomers, or homologues or mutants thereof, may be derived from any organism. The analyte may pass through the CsgG constriction, followed by the CsgF constriction. In an alternative embodiment the analyte may pass through the CsgF constriction, followed by the CsgG constriction, depending on the orientation of the CsgG/CsgF complex in the membrane.

The method is for determining the presence, absence or one or more characteristics of a target analyte. The method may be for determining the presence, absence or one or more characteristics of at least one analyte. The method may concern determining the presence, absence or one or more characteristics of two or more analytes. The method may comprise determining the presence, absence or one or more characteristics of any number of analytes, such as 2, 5, 10, 15, 20, 30, 40, 50, 100 or more analytes. Any number of characteristics of the one or more analytes may be determined, such as 1, 2, 3, 4, 5, 10 or more characteristics.

The binding of a molecule in the channel of the pore complex, or in the vicinity of either opening of the channel will have an effect on the open-channel ion flow through the pore, which is the essence of “molecular sensing” of pore channels. In a similar manner to the nucleic acid sequencing application, variation in the open-channel ion flow can be measured using suitable measurement techniques by the change in electrical current (for example, WO 2000/28312 and D. Stoddart et al., Proc. Natl. Acad. Sci., 2010, 106, 7702-7 or WO 2009/077734). The degree of reduction in ion flow, as measured by the reduction in electrical current, is related to the size of the obstruction within, or in the vicinity of, the pore. Binding of a molecule of interest, also referred to as an “analyte”, in or near the pore therefore provides a detectable and measurable event, thereby forming the basis of a “biological sensor”. Suitable molecules for nanopore sensing include nucleic acids; proteins; peptides; polysaccharides and small molecules (refers here to a low molecular weight (e.g., <900 Da or <500 Da) organic or inorganic compound) such as pharmaceuticals, toxins, cytokines, and pollutants. Detecting the presence of biological molecules finds application in personalised drug development, medicine, diagnostics, life science research, environmental monitoring and in the security and/or the defence industry.

In another aspect, the isolated pore complex, or the transmembrane pore complex containing a wild type or modified E. coli CsgG nanopore, or homologue or mutant thereof, and a modified CsgF peptide providing a channel constriction to the pore within the complex, may serve as a molecular or biological sensor. In some embodiments, the CsgG nanopore can be derived or isolated from bacterial proteins (e.g., E. coli, Salmonella typhi). In some embodiments, the CsgG nanopore can be recombinantly produced. Procedures for analyte detection are described in Howorka et al. Nature Biotechnology (2012) Jun. 7; 30(6):506-7. The analyte molecule that is to be detected may bind to either face of the channel, or within the lumen of the channel itself. The position of binding may be determined by the size of the molecule to be sensed.

The target analyte is preferably a metal ion, an inorganic salt, a polymer, an amino acid, a peptide, a polypeptide, a protein, a nucleotide, an oligonucleotide, a polynucleotide, a polysaccharide, a dye, a bleach, a pharmaceutical, a diagnostic agent, a recreational drug, an explosive, a toxic compound, or an environmental pollutant. The method may concern determining the presence, absence or one or more characteristics of two or more analytes of the same type, such as two or more proteins, two or more nucleotides or two or more pharmaceuticals. Alternatively, the method may concern determining the presence, absence or one or more characteristics of two or more analytes of different types, such as one or more proteins, one or more nucleotides and one or more pharmaceuticals.

The target analyte can be secreted from cells. Alternatively, the target analyte can be an analyte that is present inside cells such that the analyte must be extracted from the cells before the method can be carried out.

A wild-type pore may act as sensor, but is often modified via recombinant or chemical methods to increase the strength of binding, the position of binding, or the specificity of binding of the molecule to be sensed. Typical modifications include addition of a specific binding moiety complimentary to the structure of the molecule to be sensed. Where the analyte molecule comprises a nucleic acid, this binding moiety may comprise a cyclodextrin or an oligonucleotide; for small molecules this may be a known complimentary binding region, for example the antigen binding portion of an antibody or of a non-antibody molecule, including a single chain variable fragment (scFv) region or an antigen recognition domain from a T-cell receptor (TCR); or for proteins, it may be a known ligand of the target protein. In this way the wild type or modified E. coli CsgG nanopore, or homologue thereof, may be rendered capable of acting as a molecular sensor for detecting presence in a sample of suitable antigens (including epitopes) that may include cell surface antigens, including receptors, markers of solid tumours or haematologic cancer cells (e.g. lymphoma or leukaemia), viral antigens, bacterial antigens, protozoal antigens, allergens, allergy related molecules, albumin (e.g. human, rodent, or bovine), fluorescent molecules (including fluorescein), blood group antigens, small molecules, drugs, enzymes, catalytic sites of enzymes or enzyme substrates, and transition state analogues of enzyme substrates. As described above, modifications may be achieved using known genetic engineering and recombinant DNA techniques. The positioning of any adaptation would be dependent on the nature of the molecule to be sensed, for example, the size, three-dimensional structure, and its biochemical nature. The choice of adapted structure may make use of computational structural design. Determination and optimization of protein-protein interactions or protein-small molecule interactions can be investigated using technologies such as a BIAcore® which detects molecular interactions using surface plasmon resonance (BIAcore, Inc., Piscataway, N.J.; see also www.biacore.com).

In one embodiment, the analyte is an amino acid, a peptide, a polypeptides or protein. The amino acid, peptide, polypeptide or protein can be naturally-occurring or non-naturally-occurring. The polypeptide or protein can include within them synthetic or modified amino acids. Several different types of modification to amino acids are known in the art. Suitable amino acids and modifications thereof are above. It is to be understood that the target analyte can be modified by any method available in the art.

In another embodiment, the analyte is a polynucleotide, such as a nucleic acid, which is defined as a macromolecule comprising two or more nucleotides. Nucleic acids are particularly suitable for nanopore sequencing. The naturally-occurring nucleic acid bases in DNA and RNA may be distinguished by their physical size. As a nucleic acid molecule, or individual base, passes through the channel of a nanopore, the size differential between the bases causes a directly correlated reduction in the ion flow through the channel. The variation in ion flow may be recorded. Suitable electrical measurement techniques for recording ion flow variations are described in, for example, WO 2000/28312 and D. Stoddart et al., Proc. Natl. Acad. Sci., 2010, 106, pp 7702-7 (single channel recording equipment); and, for example, in WO 2009/077734 (multi-channel recording techniques). Through suitable calibration, the characteristic reduction in ion flow can be used to identify the particular nucleotide and associated base traversing the channel in real-time. In typical nanopore nucleic acid sequencing, the open-channel ion flow is reduced as the individual nucleotides of the nucleic sequence of interest sequentially pass through the channel of the nanopore due to the partial blockage of the channel by the nucleotide. It is this reduction in ion flow that is measured using the suitable recording techniques described above. The reduction in ion flow may be calibrated to the reduction in measured ion flow for known nucleotides through the channel resulting in a means for determining which nucleotide is passing through the channel, and therefore, when done sequentially, a way of determining the nucleotide sequence of the nucleic acid passing through the nanopore. For the accurate determination of individual nucleotides, it has typically required for the reduction in ion flow through the channel to be directly correlated to the size of the individual nucleotide passing through the constriction (or “reading head”). It will be appreciated that sequencing may be performed upon an intact nucleic acid polymer that is ‘threaded’ through the pore via the action of an associated polymerase, for example. Alternatively, sequences may be determined by passage of nucleotide triphosphate bases that have been sequentially removed from a target nucleic acid in proximity to the pore (see for example WO 2014/187924).

The polynucleotide or nucleic acid may comprise any combination of any nucleotides. The nucleotides can be naturally occurring or artificial. One or more nucleotides in the polynucleotide can be oxidized or methylated. One or more nucleotides in the polynucleotide may be damaged. For instance, the polynucleotide may comprise a pyrimidine dimer. Such dimers are typically associated with damage by ultraviolet light and are the primary cause of skin melanomas. One or more nucleotides in the polynucleotide may be modified, for instance with a label or a tag, for which suitable examples are known by a skilled person. The polynucleotide may comprise one or more spacers. A nucleotide typically contains a nucleobase, a sugar and at least one phosphate group. The nucleobase and sugar form a nucleoside. The nucleobase is typically heterocyclic. Nucleobases include, but are not limited to, purines and pyrimidines and more specifically adenine (A), guanine (G), thymine (T), uracil (U) and cytosine (C). The sugar is typically a pentose sugar. Nucleotide sugars include, but are not limited to, ribose and deoxyribose. The sugar is preferably a deoxyribose. The polynucleotide preferably comprises the following nucleosides: deoxyadenosine (dA), deoxyuridine (dU) and/or thymidine (dT), deoxyguanosine (dG) and deoxycytidine (dC). The nucleotide is typically a ribonucleotide or deoxyribonucleotide. The nucleotide typically contains a monophosphate, diphosphate or triphosphate. The nucleotide may comprise more than three phosphates, such as 4 or 5 phosphates. Phosphates may be attached on the 5′ or 3′ side of a nucleotide. The nucleotides in the polynucleotide may be attached to each other in any manner. The nucleotides are typically attached by their sugar and phosphate groups as in nucleic acids. The nucleotides may be connected via their nucleobases as in pyrimidine dimers. The polynucleotide may be single stranded or double stranded. At least a portion of the polynucleotide is preferably double stranded. The polynucleotide is most preferably ribonucleic nucleic acid (RNA) or deoxyribonucleic acid (DNA). In particular, said method using a polynucleotide as an analyte alternatively comprises determining one or more characteristics selected from (i) the length of the polynucleotide, (ii) the identity of the polynucleotide, (iii) the sequence of the polynucleotide, (iv) the secondary structure of the polynucleotide and (v) whether or not the polynucleotide is modified.

The polynucleotide can be any length (i). For example, the polynucleotide can be at least 10, at least 50, at least 100, at least 150, at least 200, at least 250, at least 300, at least 400 or at least 500 nucleotides or nucleotide pairs in length. The polynucleotide can be 1000 or more nucleotides or nucleotide pairs, 5000 or more nucleotides or nucleotide pairs in length or 100000 or more nucleotides or nucleotide pairs in length. Any number of polynucleotides can be investigated. For instance, the method may concern characterising 2, 3, 4, 5, 6, 7, 8, 9, 10, 20, 30, 50, 100 or more polynucleotides. If two or more polynucleotides are characterised, they may be different polynucleotides or two instances of the same polynucleotide. The polynucleotide can be naturally occurring or artificial. For instance, the method may be used to verify the sequence of a manufactured oligonucleotide. The method is typically carried out in vitro.

Nucleotides can have any identity (ii), and include, but are not limited to, adenosine monophosphate (AMP), guanosine monophosphate (GMP), thymidine monophosphate (TMP), uridine monophosphate (UMP), 5-methylcytidine monophosphate, 5-hydroxymethylcytidine monophosphate, cytidine monophosphate (CMP), cyclic adenosine monophosphate (cAMP), cyclic guanosine monophosphate (cGMP), deoxyadenosine monophosphate (dAMP), deoxyguanosine monophosphate (dGMP), deoxythymidine monophosphate (dTMP), deoxyuridine monophosphate (dUMP), deoxycytidine monophosphate (dCMP) and deoxymethylcytidine monophosphate. The nucleotides are preferably selected from AMP, TMP, GMP, CMP, UMP, dAMP, dTMP, dGMP, dCMP and dUMP. A nucleotide may be abasic (i.e. lack a nucleobase). A nucleotide may also lack a nucleobase and a sugar (i.e. is a C3 spacer). The sequence of the nucleotides (iii) is determined by the consecutive identity of following nucleotides attached to each other throughout the polynucleotide strain, in the 5′ to 3′ direction of the strand.

The pores comprising a CsgG pore and CsgF peptides are particularly useful in analysing homopolymers. For example, the pores may be used to determine the sequence of a polynucleotide comprising two or more, such as at least 3, 4, 5, 6, 7, 8, 9 or 10, consecutive nucleotides that are identical. For example, the pores may be used to sequence a polynucleotide comprising a polyA, polyT, polyG and/or polyC region.

The CsgG pore constriction is made of the residues at the 51, 55 and 56 positions of SEQ ID NO: 3. The reader head of CsgG and its constriction mutants are generally sharp. When DNA is passing through the constriction, interactions of approximately 5 bases of DNA with the reader head of the pore at any given time dominate the current signal. Although these sharper reader heads are very good in reading mixed sequence regions of DNA (when A, T, G and C are mixed), the signal becomes flat and lack information when there is a homopolymeric region within the DNA (eg: polyT, polyG, polyA, polyC). Because 5 bases dominate the signal of the CsgG and its constriction mutants, its difficult to discriminate photopolymers longer than 5 without using additional dwell time information. However, if DNA is passing through a second reader head, more DNA bases will interact with the combined reader heads, increasing the length of the homopolymers that can be discriminated. The Examples and Figures show that such an increase in homopolymer sequencing accuracy is achieved using the pore comprising a CsgG pore and a CsgF peptide.

Kit

In a further aspect, the present invention also provides a kit for characterising a target polynucleotide. The kit comprises an isolated pore complex according to the invention, and the components of a membrane or an insulating layer. The membrane is preferably formed from the components. The isolated pore complex is preferably present in the membrane or in the insulating layer, together forming a transmembrane pore complex channel. The kit may comprise components of any type of membranes, such as an amphiphilic layer or a triblock copolymer membrane. The kit may further comprise a polynucleotide binding protein. The kit may further comprise one or more anchors for coupling the polynucleotide to the membrane. The kit may additionally comprise one or more other reagents or instruments which enable any of the embodiments mentioned above to be carried out. Such reagents or instruments include one or more of the following: suitable buffer(s) (aqueous solutions), means to obtain a sample from a subject (such as a vessel or an instrument comprising a needle), means to amplify and/or express polynucleotides or voltage or patch clamp apparatus. Reagents may be present in the kit in a dry state such that a fluid sample resuspends the reagents. The kit may also, optionally, comprise instructions to enable the kit to be used in the method of the invention or details regarding for which organism the method may be used. Finally, the kit may also comprise additional components useful in peptide characterization.

In one embodiment, the isolated pore complex or transmembrane pore complex, as provided by the invention, is used for nucleic acid sequencing. For said use, the Phi29 DNA polymerase (DNAP) may be used as a molecular motor with a CsgG:CsgF Nanopore complex located within a membrane to allow controlled movement of an oligomeric probe DNA strand through the pore. A voltage may be applied across the pore and a current generated from the movement of ions in a salt solution on either side of the nanopore. As the probe DNA moves through the pore, the ionic flow through the pore changes with respect to the DNA. This information has been shown to be sequence dependent and allows for the sequence of the probe to be read with accuracy from current measurements.

It is to be understood that although particular embodiments, specific configurations as well as materials and/or molecules, have been discussed herein for engineered cells and methods according to the present invention, various changes or modifications in form and detail may be made without departing from the scope and spirit of this invention. The following examples are provided to better illustrate particular embodiments, and they should not be considered limiting the application. The application is limited only by the claims.

Examples Introduction

The CsgG pore is part of the multi-component type VIII secretion system, also known as the curli biosynthesis system, which is responsible for the formation of aggregative fibres known in E. coli as curli. Curli are extracellular proteinaceous fibres primarily involved in bacterial biofilm formation and attachment to non-biotic surfaces. Curli biosynthesis is directed by two operons, in E. coli called csgBAC and csgDEFG (curli-specific genes) (Hammar et al., 1995). Secretion of curli subunits CsgA and CsgB depends on CsgG, a dedicated lipoprotein found to form an oligomeric secretion channel in the outer membrane. For transport, CsgG works in coordination with the periplasmic and extracellular accessory proteins CsgE and CsgF. CsgE forms a specificity factor for CsgG-mediated transport, whilst CsgF appears to couple CsgA secretion to the CsgB-templated aggregation into extracellular fibers.

A crystal structure of the CsgG secretion channel demonstrated CsgG to form a nonameric transport complex 120 Å in diameter and 85 Å in height that traverses the OM through a 36-stranded β-barrel of 40 Å inner diameter (Goyal et al., 2014; FIG. 1). A periplasmic domain of the channel, separated from the transmembrane R-barrel by an iris-like diaphragm, forms a large solvent-accessible cavity of 24,000 Å3. This diaphragm is formed by a conserved 12-residue ‘constriction loop’ (CL) found in each subunit, and whose concentric organization in the CsgG oligomer forms an orifice with solvent-excluded diameter of ˜0.6 nm and a height of ˜1.5 nm (FIG. 1). When acting as a protein secretion channel, this orifice or constriction in the CsgG channel forms the primary site of interaction with the translocating polypeptide. When CsgG is used as a nanopore sensing platform, this orifice serves the role of primary reader head for analytes residing in or passing the channel. The diameter of the orifice as well as its physical and chemical properties can be modified by amino acid substitutions, deletions or insertions in a region of the protein corresponding to residues 46 to 61 of SEQ ID NO:3 (FIG. 1D). In particular, mutation of positions 51, 55, and 56 according to SEQ ID NO:3, together or individually, has a beneficial influence on the conductance characteristics of the nanopore and its interaction with analytes including polynucleotides.

The assembly factor CsgF represents a component to the curli secretion apparatus. The CsgF preprotein reaches the periplasm via the SEC pathway, after which mature CsgF (12.9 KDa) is found as a surface-exposed protein in a CsgG-dependent manner. In presence of CsgG, CsgF fractionates with the OM, and co-immunoprecipitation experiments suggested the two proteins are in direct contact. Available data demonstrate that CsgF is non-essential for productive subunit secretion, but rather suggest the protein forms a coupling factor between CsgA secretion and extracellular polymerization into curli fibers by coordinating or chaperoning the nucleating function of the CsgB subunit.

Example 1: CsgG:CsgF Complex Protein Production (Co-Expression, In Vitro Reconstitution, Coupled In-Vitro Transcription and Translation and Reconstitution of CsgG with CsgF Synthetic Peptides)

To produce the CsgG:CsgF complex, both proteins can be co-expressed in a suitable Gram-negative host such as E. coli, and extracted and purified as a complex from the outer membrane. The in vivo formation of the CsgG pore and the CsgG:CsgF complex requires targeting of the proteins to the outer membrane. To do so, CsgG is expressed as a prepro-protein with a lipoprotein signal peptide (Juncker et al. 2003, Protein Sci. 12(8): 1652-62) and Cys residue at the N-terminal position of the mature protein (SEQ ID No:3). An example of such lipoprotein signal peptide is residues 1-15 of full length E. coli CsgG as shown in SEQ ID No:2. Processing of prepro CsgG results in cleavage of the signal peptide and lipidatation of mature CsgG, following by transfer of the mature lipoprotein to the outer membrane, where it inserts as an oligomeric pore (Goyal et al. 2014, Nature 516(7530):250-3). To form the CsgG:CsgF complex, CsgF can be co-expressed with CsgG and targeted to the periplasm by means of a leader sequence such as the native signal peptide corresponding to residues 1-19 of SEQ ID No:5. CsgG:CsgF combination pores can then be extracted from the outer membrane using detergents, and purified to a homogeneous complex by chromatography (FIG. 2).

Alternatively, the CsgG:CsgF pore complex can be produced by in vitro reconstitution using the CsgG pore and CsgF—see below and FIG. 3.

For in vivo CsgG:CsgF complex formation in the example shown in FIG. 2, E. coli CsgF (SEQ ID NO:5) and CsgG (SEQ ID NO:2) were co-expressed using their native signal peptides to ensure periplasmic targeting of both proteins, as well as N-terminal lipidation of CsgG. Additionally, for ease of purification, CsgF was modified by introduction of a C-terminally 6× histidine tag and CsgG was fused C-terminally to a Strep-II tag. Co-expression and complex purification was performed as described in the Methods. SDS-PAGE analysis of the His affinity purification eluate revealed the enrichment of CsgF-His, as well as the co-purification of CsgG-Strep, suggesting the latter was in a complex with CsgF (FIG. 2B). Additionally, the SDS-PAGE revealed that a significant fraction of the eluted CsgF ran at lower molecular mass due to the loss of a N-terminal fragment of the protein (FIG. 2B, indicated with asterisk). SDS-PAGE analysis of the pooled fractions of the His-trap elution of the second affinity purification revealed the presence of CsgG and CsgF in an apparent equimolar concentrations, as well as the loss of the CsgF truncation fragment seen in the His-trap eluate (FIG. 2B). Co-elution of CsgF in the Strep-affinity purification indicated that the protein is present as a non-covalent complex with CsgG. Strikingly, the N-terminal truncation fragment of CsgF was lost in the Strep-affinity purification, suggesting that the CsgF N-terminus is required to bind CsgG (FIG. 2B).

FIG. 13 shows another example of CsgG:CsgF complex formation by in vivo co-expression. In this example, CsgG protein is modified with a C terminal Strep-II tag and the CsgF full length protein is modified with a C terminal 10× Histidine tag. Co-expressed CsgG:CsgF complex is been purified away from its constituent components by Strep tag purification followed by a Histidine tag purification as shown in the materials and methods section for characterisation of analytes. Due to the differences in molecular weights, CsgG:CsgF complex can be clearly differentiated from the CsgG pore in SDS-PAGE analysis (FIG. 13A). As shown in FIG. 13.B, two tag purification method can be successfully applied to purify the CsgG:CsgF complex away from its constituent components.

To produce the CsgG:CsgF complex by in vitro reconstitution, CsgG and CsgF were expressed in separate E. coli cultures transformed with pPG1 and pNA101, respectively, and purified, followed by in vitro reconstitution of the CsgG:CsgF complex (see Methods). For comparison, purified CsgG was similarly run over the Superose 6 column as the complex. The CsgG Superose 6 run showed the existence of two discrete populations, corresponding to nonameric CsgG pores (FIGS. 3A (a) and 3C) as well as dimers of nonameric CsgG pores (FIGS. 3A (b) and 3C), as previously described in Goyal et al. (2014). The Superose 6 run of the CsgG:CsgF reconstitution revealed the existence of three discrete populations corresponding to excess CsgF (FIG. 3A (c)), nonameric CsgG:CsgF complex (FIG. 3A (d)) and dimers of nonameric CsgG:CsgF (FIG. 3A (e)). To provide independent confirmation of the formation of CsgG:CsgF complexes, the various Superose 6 elution peaks were analysed on native PAGE (FIG. 3B).

Surprisingly, CsgG:CsgF complex can also be made by coupled in vitro transcription and translation (IVTT) method as described in the materials and methods section for characterisation of analytes. The complex can be made either by expressing CsgG and CsgF proteins in the same IVTT reaction or reconstituting separately made CsgG and CsgF in two different IVTT reactions. In the example shown in FIG. 14, E. coli T7-S30 extract system for circular DNA (Promega) has been used to make the CsgG:CsgF complex in one reaction mixture and proteins were analysed on SDS-PAGE. Since the protein expression in IVTT does not use the natural molecular machinery of protein expression, DNA that are used to express proteins in IVTT lack the DNA encoding the signal peptide region. When the DNA of CsgG is expressed in IVTT in the absence of DNA of CsgF, only the monomers of CsgG can be produced. Surprisingly, these expressed monomers can be assembled into CsgG oligomeric pores in situ by using cell extract membranes present in the IVTT reaction mixture (Figure, 14, lane 1). Although the oligomer of CsgG is SDS stable, it breaks down into its constituent monomers when the sample is heated to 100° C. (Figure, 14, lane 2). When the DNA of CsgF is expressed in IVTT in the absence of DNA of CsgG, only CsgF monomers can be seen (Figure, 14, lane 3). When DNA of CsgG and CsgF are mixed in 1:1 ratio and expressed simultaneously in the same IVTT reaction mixture, CsgF proteins generated interact with the assembled CsgG pore with high efficiency to make CsgG:CsgF complex (Figure, 14, lane 5). This SDS stable complex made in IVTT is heat stable at least up to 70° C. (Figure, 14, lanes 6-12).

CsgG:CsgF complexes with truncated CsgF can also be made by any of the methods shown above by using DNA encoding truncated CsgF instead of the full length version. However, stability of the complex may be compromised when CsgF is truncated below the FCP domain. In addition, CsgG:CsgF complexes with truncated CsgF can be made by cleaving the full length CsgF in appropriate positions once the full length CsgG:CsgF complex is formed. Truncations can be done by modifying the DNA that encode CsgF protein by incorporating protease cleavage sites at positions where cleavage is needed (FIG. 15.A). Seq ID No. 56-67 show TEV or HCV C3 protease sites incorporated in various positions of CsgF to generate CsgG:CsgF complexes with truncated CsgF. SDS-PAGE analysis of TEV cleavage of the CsgG:CsgF complex made with the Seq ID No. 61 is shown (FIG. 15.B). When the CsgG:CsgF complex (with full length CsgF) is treated with TEV protease enzyme as described in the materials and methods section for characterisation of analytes, CsgF is being truncated at position 35 (FIG. 15.B, lanes 3 and 4). However, TEV cleavage leaves an extra 6 amino acids at the C terminal of the cleavage site. Therefore, remaining CsgF truncated protein in complex with the CsgG pore is 42 amino acids long. Molecular weight difference of this complex and the CsgG pore (without the CsgF) is still visible in SDS-PAGE (FIG. 15.B, lanes 7 and 8).

Surprisingly, CsgG:CsgF complexes with truncated CsgF can also be made by reconstituting purified CsgG pore (made by in vivo or in vitro) with synthetic peptides of appropriate length. Since the reconstitution takes place in vitro, signal peptide of CsgF is not required to make the CsgG:CsgF complex. Further, this method does not leave extra amino acids at the C terminus of the CsgF. Mutations and modifications can also be easily incorporated into synthetic CsgF peptides. Therefore, this method is a very convenient way to reconstitute different CsgG pores or mutants or homologues thereof with different CsgF peptides or mutants or homologues thereof to generate different CsgG:CsgF complex variants. Stability of the complex may be compromised when the CsgF is truncated beyond the FCP domain. Examples of truncated CsgF and FCP peptides used to generate CsgG:CsgF complex variants are shown in Table 3. Surprisingly, SDS-PAGE analysis of the heat stability of CsgG:CsgF complexes made by this method with CsgF-(1-45) (FIG. 16.A), CsgF-(1-35) (FIG. 16.B) and CsgF-(1-30) (FIG. 16.C) shows at least CsgF-(1-45) and CsgF-(1-35) peptides make complexes with CsgG that are heat stable at least to 90° C. Since the CsgG pore breaks down to its constituent monomers at 90° C., it is difficult to assess the stability of the complex beyond 90° C. Due to the minimal difference between the CsgG pore band and the CsgG:CsgF-(1-30) complex band in SDS-PAGE, this method is not sufficient to analyse the heat stability of the CsgG:CsgF-(1-30) complex (FIG. 16.C). However, CsgG:CsgF complexes have been observed in all three cases and even with CsgG:CsgF-(1-29) in electrophysiological experiments indicating that even CsgF-(1-29) peptide is producing at least some CsgG:CsgF complexes (FIG. 24).

Example 2: CsgG:CsgF Structural Analysis Via Cryo-EM

To gain structural insight in the CsgG:CsgF complex, co-purified or in vitro reconstituted CsgG:CsgF particles were analysed by transmission electron microscopy. In preparation of cryo-EM analysis, 500 μL of the peak fraction of the double-affinity purified CsgG:CsgF complex was injected onto a Superose 6 10/30 column (GE Healthcare) equilibrated with Buffer D (25 mM Tris pH8, 200 mM NaCl and 0.03% DDM), and run at 0.5 mL/min. Protein concentration was determined based on calculated absorbance at 280 nm and assuming 1:1 stoichiometry. Samples for electron cryomicroscopy were analysed as described in the Methods. A cryo-EM micrograph of the CsgG:CsgF complex as well as two selected class averages from the picked CsgG:CsgF particles are shown in FIG. 4. The micrograph shows the presence of nonameric pore as well as dimer of nonameric pore complexes. For image reconstruction, nonameric CsgG:CsgF particles were picked and aligned using RELION. Class averages of the CsgG:CsgF complex as side views, as well as the 3D reconstructed electron density show the presence of an additional density corresponding to CsgF, seen as a protrusion from the CsgG particle, located at the side of the CsgG β-barrel (FIG. 4B, 5). The additional density reveals three distinct regions, encompassing a globular head domain, a hollow neck domain and a domain that interacts with the CsgG β-barrel. The latter CsgF region, referred to as CsgF constriction peptide or FCP, inserts into the lumen of the CsgG β-barrel and can be seen to form an additional constriction (labeled F in FIG. 4B, 5) of the CsgG pore, located approximately 2 nm above the constriction formed by the CsgG constriction loop (labeled G in FIG. 4B, 5).

Example 3: Identification of the CsgF Interaction and Constriction Peptide by Truncation of CsgF

The presence of a second constriction in the CsgG:CsgF pore complex as compared to the CsgG only pore provides opportunities for nanopore sensing applications, providing a second orifice in the nanopore that can be used as a second reader head or as an extension of the primary reader head provided by the CsgG constriction loop (FIG. 6, 7). However, when in complex with the full length CsgF, the exit side of CsgG:CsgF combination pore is blocked by the CsgF neck and head domains. Therefore, we sought to determine the CsgF region required to interact with and insert into the CsgG β-barrel. Our Strep-tactin affinity purification experiments hinted that the N-terminal region of CsgF was required for CsgG interaction, since an N-terminal truncation fragment of CsgF present in the His-trap affinity purification was lost and did not co-purify with CsgG (FIG. 2B). CsgF homologues are characterised by the presence of PFAM domain PF03783. When performing a multiple sequence alignment (MSA) of CsgG homologues found in Gram-negative bacteria (FIG. 8 shows a MSA of a selected CsgF homologues) a region of sequence conservation (between 35 and 100% pairwise sequence identity) was seen corresponding to the first ˜30-35 amino acids of mature CsgF (SEQ ID NO:6). Based on the combined data, this N-terminal region of CsgF was hypothesised to form the CsgG interaction peptide or FCP. A multiple sequence alignment of the FCPs in known CsgF homologues is shown in FIG. 10.

To test the hypothesis that the CsgF N-terminus corresponds to the CsgG binding region and forms the CsgF constriction peptide residing in the CsgG β-barrel lumen, Strep-tagged CsgG and His-tagged CsgF truncates were co-overexpressed in E. coli (see Methods). pNA97, pNA98, pNA99 and pNA100 encode N-terminal CsgF fragments corresponding to residues 1-27, 1-38, 1-48 and 1-64 of CsgF (SEQ ID NO:5). These peptides include the CsgF signal peptide corresponding to residues 1-19 of SEQ ID NO: 5, and thus will produce periplasmic peptides corresponding to the first 8, 19, 29 and 45 residues of mature CsgF (SEQ ID NO:6; FIG. 9A), each including a C-terminal 6× His tag. SDS-PAGE analysis of whole cell lysates revealed the presence of CsgG in all samples, as well as the presence of CsgF fragment corresponding to the first 45 residues of mature CsgF (SEQ ID NO:6; FIG. 9B). For the shorter N-terminal CsgF fragments, no detectible expression of the peptides was seen in the whole cell lysates. After two freeze/thaw cycles, cell mass of the various CsgG:CsgF fragments were further enriched by purification. Whole cell lysates as well as the eluted fractions of the Strep affinity purification were spotted onto a nitrocellulose membrane for dot blot analysis using an anti-His antibody for the detection of the His-tagged CsgF fragments (FIG. 9C). The dot blot shows the CsgF 20:64 peptide co-purifies with CsgG, demonstrating this CsgF fragment is sufficient to form a stable non-covalent complex with CsgG. For the CsgG 20:48 fragment a small amount of peptide can be seen to co-purify with CsgG, whilst no detectable levels are seen for CsgF 20:27 or CsgF 20:38 in either the whole cell lysate or the Strep affinity purification (FIG. 9C), suggesting that the latter peptides are not stably expressed in E. coli, and/or do not form a stable complex with CsgG.

Example 4: Description of the CsgG:CsgF Interaction at Atomic Resolution

To gain an atomic level detail on the CsgG:CsgF interaction we determined the high resolution cryoEM structure of the CsgG:CsgF complex. For this purpose, CsgG and CsgF were co-expressed recombinantly in E. coli and the CsgG:CsgF complex was isolated from E. coli outer membranes by detergent extraction and purified using tandem affinity purification. Samples for electron cryo-microscopy were prepared by spotting 3 μl sample on R2/1 Holey grids (Quantifoil), coated with graphene oxide, and data was collected on a 300 kV TITAN Krios with Gatan K2 direct electron detector in counting mode. 62.000 single CsgG:CsgF particles were used to calculate a final electron density map at 3.4 Å resolution (FIG. 11A). The map allowed unambiguous docking and local rebuilding of the CsgG crystal structure, as well as the de novo building of the N-terminal 35 residues of mature CsgF (i.e. residues 20:54 of Seq ID No. 5), which encompass the FCP that binds CsgG and forms a second constriction at the height of the CsgG transmembrane β-barrel (FIG. 11C, D). The cryoEM structure shows CsgG:CsgF comprises a 9:9 stoichiometry, with C9 symmetry (FIG. 11B). The FCP binds the inside of the CsgG β-barrel, with the C-terminus of the CsgF pointing out of the CsgG β-barrel, and the CsgF N-terminus located near the CsgG constriction. The structure shows that P35 in mature CsgF lies outside the CsgG β-barrel and forms the connection between the CsgF FCP and neck regions. The CsgF neck and head regions are not resolved in the high resolution cryoEM maps due to flexibility relative to the main body of the CsgG:CsgF complex. Three regions in the CsgG β-barrel stabilize the CsgG:CsgF interaction: (IR1) residues Y130, D155, S183, N209 and T207 in mature CsgG (SEQ ID NO: 3) form an interaction network with the N-terminal amine and residues 1 to 4 of mature CsgF (SEQ ID NO: 6), comprising four H-bonds and an electrostatic interaction; (IR2) residues Q187, D149 and E203 in mature CsgG (SEQ ID NO: 3) form an interaction network with R8 and N9 in mature CsgF (SEQ ID NO 6), encompassing three H-bonds and two electrostatic interaction; and (IR3) residues F144, F191, F193 and L199 in mature CsgG (SEQ ID NO: 3) form a hydrophobic interaction surface with residues F21, L22 and A26 in mature CsgF (SEQ ID NO: 6). The latter are located in an α-helix (helix 1) formed by residues 19-30 of mature CsgF. The conserved sequence N-P-X-F-G-G (residues 9-14 in SEQ ID NO: 6) forms an inward turn that connects the loop region formed by residues 15-19 with the CsgF helix 1. Together, these elements give rise to a constriction in the CsgG:CsgF complex, of which residue 17 (N17 in mature E. coli CsgF, SEQ ID NO: 6) forms the narrowest point, resulting in an orifice with 15 Å diameter (FIG. 11C). The second constriction (F-constriction or FC) lies approximately 15 Å and 30 Å above the top and bottom, respectively, of the constriction formed by CsgG residues 46 to 59 (G-constriction or GC).

Example 5: Simulations to Improve CcgG-CsgF Complex Stability

Molecular dynamics simulations were performed to establish which residues in CsgG and CsgF come into close proximity. This information was used to design CsgG and CsgF mutants that could increase the stability of the complex.

Simulations were performed using the GROMACS package version 4.6.5, with the GROMOS 53a6 forcefield and the SPC water model. The cryo-EM structure of the CsgG-CsgF complex was used in the simulations. The complex was solvated and then energy minimised using the steepest descents algorithm. Throughout the simulation, restraints were applied to the backbone of the complex, however, the residue sidechains were free to move. The system was simulated in the NPT ensemble for 20 ns, using the Berendsen thermostat and Berendsen barostat to 300 K.

Contacts between CsgG and CsgF were analysed using both GROMACS analysis software and also locally written code. Two residues were defined as having made a contact if they came within 3 Angstroms. The results are shown in Table 4 below.

TABLE 4 Predicted contact frequencies of residue pairs in the CsgG/CsgF complex: CsgG CsgF % Time spent residue residue in contact GLU 203 ARG 8 88.8 GLU 201 ASN 11 87.4 GLU 201 PHE 12 84.3 GLU 203 ASN 9 83.6 ASP 155 GLY 1 81.2 GLU 203 PHE 7 81 GLU 201 ASN 9 77.2 SER 183 GLY 1 76.1 ASN 209 MET 3 70.8 THR 207 PHE 5 70.1 ASP 149 ARG 8 68.5 GLN 187 ARG 8 66.1 ARG 142 PHE 12 65.4 GLU 185 ARG 8 64.4 ASP 149 PHE 12 64.2 GLN 187 GLN 6 63.3 GLY 205 PHE 5 54 GLN 197 ASN 30 52.5 GLN 197 SER 31 51.4 LYS 49 THR 2 50.8 PHE 144 GLN 29 50.6 GLU 201 PHE 21 48 GLN 151 PHE 5 47 PHE 191 ASN 9 46.9 ARG 142 ASN 11 46.4 GLN 151 PHE 7 45.6 TYR 196 TYR 32 45.4 PHE 191 PHE 21 45.3 PHE 193 ALA 26 45.1 GLU 201 SER 25 44.9 LEU 199 GLN 29 44.7 ARG 141 PHE 12 43.1 GLY 138 PHE 7 43 GLN 187 PHE 5 43 GLY 145 GLN 29 42.4 GLN 153 GLY 1 42.1 GLY 140 PHE 7 40.5 PHE 193 TYR 32 39.9 GLU 203 PHE 12 39.7 ASN 133 PHE 5 35.9 GLN 151 MET 3 32 PHE 193 ASN 30 31.9 SER 136 PHE 5 31.7 PHE 144 SER 31 30.3 TYR 130 GLY 1 30 GLN 187 PHE 7 29.9 PHE 192 ASN 30 28.9 GLY 138 PHE 5 28.3 ILE 194 TYR 32 26.7 ASN 209 GLY 1 26.1 PHE 192 GLN 29 25.8 PHE 193 GLN 29 25.4 PHE 193 GLN 27 23.9 ASP 149 GLY 13 22.7 TYR 196 ASN 30 22.6 PHE 192 SER 31 22.2 ASP 148 PHE 12 22 GLY 140 PHE 12 21.7 TYR 196 ASP 34 21.6 ARG 198 SER 31 19.9 VAL 139 PHE 7 19.5 PHE 191 ALA 26 18.3 ASN 132 GLY 1 18.1 TYR 195 TYR 32 17.9 GLN 197 ALA 28 17.6 GLN 151 ARG 8 16.9 PHE 191 LEU 22 16.5 PHE 191 GLN 29 15.6 THR 206 PHE 5 14.7 GLN 153 MET 3 14.3 PHE 192 TYR 32 13.8 GLU 201 GLN 29 13.3 ARG 142 SER 25 13.3 PHE 144 ASN 30 12.6 ARG 142 ARG 8 12.6 PHE 191 ASN 11 12.3 GLU 131 THR 2 12.2 ASN 133 GLY 1 11.3 GLY 205 PHE 7 11.2 GLN 151 PHE 12 10.4 ASN 132 PHE 5 10.3 GLU 202 PHE 12 10.2 ASP 149 PHE 7 10.2

Materials and Methods for Structural Determination of the CsgG:CsgF Complex: Cloning

For the expression of E. coli CsgG as outer membrane localized pore, the coding sequence of E. coli CsgG (SEQ ID NO:1) was cloned into pASK-Iba12, resulting in plasmid pPG1 (Goyal et al. 2013).

For the expression of C-terminally 6×-His tagged CsgF in the E. coli cytoplasm, the coding sequence for mature E. coli CsgF (SEQ ID NO:6; i.e. CsgF without its signal sequence) was cloned into pET22b via the Ndel and EcoRI sites, using a PCR product generated using the primers “CsgF-His_pET22b_FW” (SEQ ID NO:46) and “CsgF-His_pET22b_Rev” (SEQ ID NO:47), resulting in the CsgF-His expression plasmid pNA101.

The pNA62 plasmid, a pTrc99a based vector expressing csgF-His and csgG-strep, was created based on pGV5403 (pTrc99a with the pDEST14 Gateway® cassette integrated). The pGV5403 ampicillin resistance cassette was replaced by a streptomycin/spectinomycin resistance cassette. A PCR fragment encompassing part of the E. coli MC4100 csgDEFG operon corresponding to the coding sequences of csgE, csgF and csgG was generated with primers csgEFG_pDONR221_FW (SEQ ID NO:48) and csgEFG_pDONR221_Rev (SEQ ID NO:49), and inserted in pDONR221 (ThermoFisher Scientific) via BP Gateway® recombination. Next, this recombinant csgEFG operon from the pDONR221 donor plasmid was inserted via LR Gateway® recombination into pGV5403 with streptomycin/spectinomycin resistance cassette. Via PCR, a 6×His-tag was added to the CsgF C-terminus using primers Mut_csgF_His_FW (SEQ ID NO:50) and Mut_csgF_His_Rev (SEQ ID NO:51). Finally, csgE was removed by outwards PCR (primers DelCsgE_FW (SEQ ID NO:52) and DelCsgE_Rev (SEQ ID NO:53)) to obtain pNA62.

Constructs for the periplasmic expression of C-terminally His-tagged CsgF fragments corresponding to the putative constriction peptides (FIG. 9A) were created by outwards PCR on pNA62, a pTrc99a based vector expressing CsgF-his and CsgG-strep. Primer combinations were as follows: pNa62_CsgF_histag_Fw (SEQ ID NO:45) as forward primers, with CsgF_d27_end (SEQ ID NO:41), CsgF_d38_end (SEQ ID NO:42), CsgF_d48_end (SEQ ID NO:43) or CsgF_d64_end (SEQ ID NO:44) as reverse primers to create pNA97, pNA98, pNA99 and pNA100 respectively.

In pNA97 csgF is truncated to SEQ ID NO:7, encoding a CsgF fragment including residues 1-27 (SEQ ID NO:8); In pNA98 csgF is truncated to SEQ ID NO:9, encoding a CsgF fragment including residues 1-38 (SEQ ID NO:10); In pNA99 csgF is truncated to SEQ ID NO:11, encoding a CsgF fragment including residues 1-48 (SEQ ID NO:12); and in pNA100 csgF is truncated to SEQ ID NO:13, encoding a CsgF fragment including residues 1-64 (SEQ ID NO:14). Expression of pNA97, pNA98, pNA99 and pNA100 in E. coli does result in production of the CsgG pore (SEQ ID NO:3) in the outer membrane, as well as periplasmic targeting of CsgF-derived peptides with sequences:

(SEQ ID NO: 37 + 6× His) “GTMTFQFRHHHHHH”, (SEQ ID NO: 38 + 6× His) “GTMTFQFRNPNFGGNPNNGHHHHHH”, (SEQ ID NO: 39 + 6× His) “GTMTFQFRNPNFGGNPNNGAFLLNSAQAQHHHHHH”, and (SEQ ID NO: 40 + 6× His) “GTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKDPSY NDDFGIETHHHHHH”, respectively.

Strains

E. coli Top10 (F⁻ mcrA Δ(mrrhsdRMS:mcrBC) ϕ80lacZΔM15 Δ lacX74 recA1 araD139 Δ(araleu) 7697 galU galK rpsL (StrR) endA1 nupG) was used for all cloning procedures. E. coli C43(DE3) (F⁻ ompT hsdSB (rB⁻ mB⁻) gal dcm (DE3)) and Top10 were used for protein production.

Recombinant CsgG:CsgF Complex Production Via Co-Expression

For co-expression of E. coli CsgF (SEQ ID NO:5) and CsgG (SEQ ID NO:2), both recombinant genes including their native Shine Dalgarno sequences were placed under control of the inducible trc promotor in a pTrc99a-derived plasmid to form plasmid pNA62. CsgG and CsgF were overexpressed in E. coli C43(DE3) cells transformed with plasmid pNA62 and grown at 37° C. in Terrific Broth medium. When the cell culture reached an optical density (OD) at 600 nm of 0.7, recombinant protein expression was induced with 0.5 mM IPTG and left to grow for 15 hours at 28° C., before being harvested by centrifugation at 5500 g.

Recombinant CsgG:CsgF Complex Production Via In Vitro Reconstitution

Full-length E. coli CsgG (SEQ ID NO:2) modified with a C-terminal StrepII-tag was overexpressed in E. coli BL21 (DE3) cells transformed with plasmid pPG1 (Goyal et al. 2013). The cells were grown at 37° C. to an OD 600 nm of 0.6 in Terrific Broth medium. Recombinant protein production was induced with 0.0002% anhydrotetracyclin (Sigma) and the cells were grown at 25° C. for a further 16 h before being harvested by centrifugation at 5500 g.

E. coli CsgF (SEQ ID NO:6; i.e. lacking the CsgF signal sequence) in a C-terminal fusion with a 6× His-tag was overexpressed in the cytoplasm of E. coli BL21(DE3) cells transformed with plasmid pNA101. Cells were grown at 37° C. to an OD of 600 nm followed by induction by 1 mM IPTG and left to express protein 15h at 37° C. before being harvested by centrifugation at 5500 g.

Recombinant Protein Purification of the CsgG:CsgF Complex, CsgG, and CsgF

E. coli cells transformed with pNA62 and co-expressing CsgG-Strep and CsgF-His were resuspended in 50 mM Tris-HCl pH 8.0, 200 mM NaCl, 1 mM EDTA, 5 mM MgCl₂, 0.4 mM AEBSF, 1 μg/mL Leupeptin, 0.5 mg/mL DNase I and 0.1 mg/mL lysozyme. The cells were disrupted at 20 kPsi using a TS series cell disruptor (Constant Systems Ltd) and the lysed cell suspension incubated 30′ with 1% n-dodecyl-β-d-maltopyranoside (DDM; Inalco) for further cell lysis and extraction of outer membrane components. Next, remaining cell debris and membranes were spun down by ultracentrifugation at 100.000 g for 40′. Supernatant was loaded onto a 5 mL HisTrap column equilibrated in buffer A (25 mM Tris pH8, 200 mM NaCl, 10 mM imidazole, 10% sucrose and 0.06% DDM). Column was washed with >10 CVs 5% buffer B (25 mM Tris pH8, 200 mM NaCl, 500 mM imidazole, 10% sucrose and 0.06% DDM) ion buffer A and eluted with a gradient of 5-100% buffer B over 60 mL.

Eluens was diluted 2-fold before loading overnight on a 5 mL Strep-tactin column (IBA GmbH) equilibrated with buffer C (25 mM Tris pH8, 200 mM NaCl, 10% sucrose and 0.06% DDM). Column was washed with >10 CVs buffer C and protein was eluted by the addition of 2.5 mM desthiobiotin. Next 500 μL of the peak fraction of the double-affinity purified complex was injected on a Superose 6 10/30 (GE Healthcare) equilibrated with Buffer D (25 mM Tris pH8, 200 mM NaCl and 0.03% DDM), and run at 0.5 mL/min to prepare samples for electron microscopy. Protein concentration was determined based on calculated absorbance at 280 nm and assuming 1/1 stoichiometry. Buffer D (25 mM Tris pH8, 200 mM NaCl and 0.03% DDM)

CsgG-strep purification for in vitro reconstitution is identical to the protocol for CsgG:CsgF when omitting sucrose in the buffers and bypassing the IMAC and size exclusion steps.

CsgF-His purification for in vitro reconstitution was performed by resuspension of the cell mass in 50 mM Tris-HCl pH 8.0, 200 mM NaCl, 1 mM EDTA, 5 mM MgCl2, 0.4 mM AEBSF, 1 μg/mL Leupeptin, 0.5 mg/mL DNase I and 0.1 mg/mL lysozyme. The cells were disrupted at 20 kPsi using a TS series cell disruptor (Constant Systems Ltd) and the lysed cell suspension was centrifuged at 10.000 g for 30 min to remove intact cells and cell debris. Supernatant was added to 5 mL Ni-IMAC-beads (Workbeads 40 IDA, Bio-Works Technologies AB) equilibrated with buffer A (25 mM Tris pH8, 200 mM NaCl, 10 mM imidazole) and left incubating for 1 hour at 4° C. Ni-NTA beads were pooled in a gravity flow column and washed with 100 mL of 5% buffer B (25 mM Tris pH8, 200 mM NaCl, 500 mM imidazole diluted in buffer A. Bound protein was eluted by stepwise increase of Buffer B (10% steps of each 5 mL).

In Vitro Reconstitution of the CsgG:CsgF Complex

Purified CsgG and CsgF were pooled and used to in vitro reconstitute the complex. Therefore a molar ratio of 1 CsgG:2 CsgF was mixed to saturate the CsgG barrel with CsgF. Next, the reconstituted mixture was injected on a Superose 6 10/30 column (GE Healthcare) equilibrated with Buffer D (25 mM Tris pH8, 200 mM NaCl and 0.03% DDM), and run at 0.5 mL/min to prepare samples for electron microscopy (FIG. 3). Protein concentration was determined based on calculated absorbance at 280 nm and assuming 1/1 stoichiometry.

Structural Analysis Using Electron Microscopy

Sample behavior of the size exclusion fraction is probed using negative stain electron microscopy. Samples are stained with 1% uranyl formate and imaged using an in-house 120 kV JEM 1400 (JEOL) microscope equipped with a LaB6 filament. Samples for electron cryomicroscopy were prepared by spotting 2 μL sample onto R2/1 continuous carbon (2 nm) coated grids (Quantifoil), manually blotted and plunged in liquid ethane using an in house plunging device. Sample quality was screened on the in-house JEOL JEM 1400 before collecting a dataset on a 200 kV TALOS ARCTICA (FEI) microscope equipped with a Falcon-3 direct electron detection camera. Images were motion corrected with MotionCor2.1 (Zheng et al. 2017), defocus values were determined using ctffind4 (Rohou and Grigorieff, 2015) and data was further analysed using a combination of RELION (Scheres, 2012) and EMAN2 (Ludtke, 2016). C9 Symmetry was imposed during 3D model generation and refinement on selected 2D class averages featuring additional density for a head group.

For high resolution cryoEM analysis, CsgG:CsgF samples were prepared for electron cryo-microscopy by spotting 3 μl sample on R2/1 Holey grids (Quantifoil), coated with graphene oxide (Sigma Aldrich), manually blotted and plunged in liquid ethane using CP3 plunger (Gatan). Sample quality was screened on the in-house JEOL JEM 1400 before collecting a dataset on a 300 kV TITAN KRIOS (FEI, Thermo-Scientific) microscope equipped K2 Summit direct electron detector (Gatan). The detector was used in counting mode with a cumulative electron dose of 56 electrons per Å² spread over 50 frames. 2045 images were collected with a pixel size of 1.07 Å. Images were motion-corrected with MotionCor2.1 (Zheng et al. 2017) and defocus values were determined using ctffind4 (Rohou and Grigorieff, 2015). Particles were picked automatically using Gautomatch (Dr. Kai Zhang) and data was further analysed using a combination of RELION2.0 (Kimanius et al. 2016, Elife 5. pii: el 8722) and EMAN2 (Ludtke, 2016). C9 Symmetry was imposed during 3D model generation and refinement on selected 2D class averages featuring additional density for the head group corresponding to CsgF. 62.000 particles were used to calculate the final map at 3.4 Å resolution. De novo model building of CsgF was done with COOT (Brown et al. 2015 Acta Crystallogr D Biol Crystallogr 71 (Pt 1):136-53) and iterative cycles of model building and refinement of the full complex was done with PHENIX (Afonine 2018, Acta Crystallogr D Struct Biol 74(Pt 6):531-544) real-space refinement in combination with COOT.

Protein Expression and Purification of CsgG:CsgF Fragments

CsgF fragments and CsgG were co-expressed, with CsgF fragments being C-terminally His-tagged and CsgG fused C-terminally to a Strep tag. The CsgG: CsgF fragments complex was over-expressed in E. coli Top10 cells, transformed with plasmid pNA97, pNA98, pNA99 or pNA100. Plates were grown at 37° C. ON, and a colony was resuspended in LB medium supplemented with Streptomycin/spectomycin. When the cell cultures reached an optical density (OD) at 600 nm of 0.7, recombinant protein expression was induced with 0.5 mM IPTG and left to grow for 15 hours at 28° C., before being harvested by centrifugation at 5500 g. Pellets were frozen at −20° C.

Cell mass for the various CsgG:CsgF fragment co-expressions was resuspended in 200 mL 50 mM Tris-HCl pH 8.0, 200 mM NaCl, 1 mM EDTA, 5 mM MgCl2, 0.4 mM AEBSF, 1 μg/mL Leupeptin, 0.5 mg/mL DNase I and 0.1 mg/mL lysozyme, sonicated and incubated with 1% n-dodecyl-β-d-maltopyranoside (DDM; Inalco) for further cell lysis and extraction of outer membrane components. Next, remaining cell debris and membranes were spun down by centrifugation at 15.000 g for 40′. The supernatant was incubated with 100 μL Strep-tactin beads at RT for 30 min. Strep beads were washed with buffer (25 mM Tris pH8, 200 mM NaCl, and 1% DDM) by centrifugation and bound proteins were eluted by the addition of 2.5 mM desthiobiotin in 25 mM Tris pH8, 200 mM NaCl, 0.01% DDM.

Production of CsgG:FCP by In Vitro Reconstitution.

A synthetic peptide corresponding to the N-terminal 34 residues of mature CsgF (SEQ ID NO: 6) was diluted to 1 mg/ml in buffer 0.1 M MES, 0.5 M NaCl, 0.4 mg/ml EDC (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide), 0.6 mg/ml NHS (N-hydroxysuccinimide) and incubated for 15 min at room temperature to allow activation of the peptide carboxyterminus. Next, 1 mg/ml Cadaverin-Alexa594 in PBS was added during a 2h incubation to allow covalent coupling at room temperature. The reaction was quenched via buffer exchange to 50 mM Tris, NaCl, 1 mM EDTA, 0.1% DDM using Zeba Spin filters.

Labelled peptide was added to strep-affinity purified CsgG in 50 mM Tris, 100 mM NaCl, 1 mM EDTA, 5 mM LDAO/C8D4 in a 2:1 molar ratio during 15 minutes at room temperature to allow reconstitution of the CsgG:FCP complex. After pull down of CsgG-strep on StrepTactin beads, the sample was analysed on native-PAGE.

Example 6: Further Stabilization of CsgG:CsgF Complex by Covalent Cross Linking

Although full length and some of the truncated versions of CsgF make stable CsgG:CsgF complexes with the CsgG pore, CsgF can still be dislodged from the barrel region of CsgG pore under certain conditions. Therefore, it is desirable to make a covalent link between the CsgG and CsgF subunits. Based on molecular simulation studies, positions of CsgG and CsgF that are in close proximity to each other have been identified (Example 5 and Table 4). Some of these identified positions have been modified to incorporate a Cysteine in both CsgG and CsgF. FIG. 19 shows an example of thiol-thiol bond formation between Q153 position of CsgG and G1 position of CsgF. CsgG pore containing Q153C mutation was reconstituted with CsgF containing G1C mutation and incubated for 1 hour enabling S—S bond formation. When the complex is heated to 100° C. in the absence of DTT, a 45 kDa band corresponding to dimer between CsgG monomer and CsgF monomer (CsgGm-CsgFm) can be seen indicating the S—S bond formation between the two monomers (CsgGm is 30 kDa and CsgFm is 15 kDa) (FIG. 19.A). This band disappears when the heating is done in the presence of DTT. DTT breaks down the S—S bond. When the CsgG:CsgF complex incubated overnight instead of 1 hour, the extend of CsgGm-CsgFm dimer formation increases (FIG. 19.A). Mass spectroscopy methods have been carried out to further identify the dimer band. Gel purified protein was proteolytically cleaved to generate tryptic peptides. LC-MS/MS sequencing methods were performed, resulting in the identification of S—S bond between the Q153 position of CsgG and G1 position of CsgF (FIG. 19.B). Oxidising agents such as copper-orthophenanthroline can be used to enhance the S—S bond formation. When CsgG pore containing N133C modification is reconstituted with CsgF containing T4C modification in the presence of copper-orthophenanthroline as described in methods section and then broken down to its constituent monomers by heating to 100° C. in the absence of DTT, a strong dimer band corresponding to CsgGm-CsgFm can be observed on SDS-PAGE (FIG. 20, lanes 3 and 4). When the heating was carried out in the presence of DTT, the dimer breaks down to its constituent monomers (FIG. 20, lanes 1 and 2).

Example 7: Electrophysiological Characterisation of CsgG:CsgF Complexes

The signal observed when a DNA strand translocates through CsgG is well characterised when the pore is inserted in the copolymer membrane and experiments are carried out using the MinION of Oxford Nanopore Technologies (FIG. 31). Y51, N55 and F56 of each subunit of CsgG form the constriction of the CsgG pore (FIG. 12). This sharp constriction serves as the reader head of the CsgG pore (FIG. 31A) and is able to accurately discriminate a mixed sequence of A,C,G and T as it passes through the pore. This is because the measured signal contains characteristic current deflections from which the identity of the sequence can be derived. However, in homopolymeric regions of DNA, the measured signal may not show current deflections of sufficient magnitude to allow single base identification; such that an accurate determination of the length of a homopolymer cannot be made from the magnitude of the measured signal alone (FIGS. 26 B and C). The reduction in accuracy of the CsgG reader head is correlated to the length of the homopolymeric region (FIG. 29 C). When CsgF interacts with the CsgG pore to make the CsgG:CsgF complex, CsgF introduces a second reader head within the CsgG barrel. This second reader head primarily consists of the N17 position of Seq. ID No. 6. A static strand experiment as described in the methods section and FIG. 27 was carried out to map the two reader heads of the CsgG:CsgF complex experimentally, and results indicate the presence of the two reader heads that are separated from each other by approximately 5-6 bases (FIG. 27, B, C and D). Reader head discrimination plot for the CsgG:CsgF complex shows that the second reader head introduced by CsgF contributes less to the base discrimination than that of the CsgG reader head (FIG. 27A). Surprisingly, when a second reader head is introduced by CsgF within the CsgG barrel, the homopolymeric region which was flat previously shows a step wise signal (FIGS. 30 B and C). These steps contain information that can be used to identify the sequence accurately resulting in a decrease in errors. Accuracy of the DNA signal of the CsgG:CsgF complex remains relatively constant over a longer homopolymeric length compared to the accuracy profile of the CsgG pore by itself (FIG. 29 C).

CsgG:CsgF complexes made in any of the methods described in the methods section can be used to characterise the complex in DNA sequencing experiments. Signals of a lambda DNA strand passing through various CsgG:CsgF complexes made by different methods consisting of different CsgG mutant pores and different CsgF peptides with different lengths are shown in FIGS. 21-24. Reader head discrimination of those pore complexes and their base contribution profiles are shown in FIG. 28 (A-H). Surprisingly, different modifications at constrictions of both CsgG pore and the CsgF peptide can alter the signal of the CsgG:CsgF pore complex significantly. For example, when the CsgG:CsgF complexes are made with the same CsgG pore, but with two different CsgF peptides of the same length containing either Asn or Ser at position 17 (of Seq ID No. 6) (made by the same method of co-expression of the full length CsgF protein followed by TEV protease cleavage of CsgF between positions 35 and 36), the signals generated are different from each other (FIG. 21). The CsgG:CsgF complex with Ser at position 17 of the CsgF peptide shows lower noise and higher signal:noise ratio compared to the CsgG:CsgF complex with Asn at position 17 of the CsgF peptide. Similarly, when the same CsgG pore was reconstituted with two different peptides of CsgF of the same length (1-35 of Seq ID No. 6) but with either Ser or Val at positon 17 to make the CsgG:CsgF complexes, the complex with Val at position 17 of CsgF shows a noisier signal than the complex with Ser at position 17 of CsgF (FIG. 22). When the same CsgF peptide of the same length was reconstituted with different CsgG pores containing different mutations at the CsgG reader head (positions 51, 55 and 56), the resulting CsgG:CsgF complexes showed very different signals (FIG. 23, A-F) with different signal to noise ratios (FIG. 25). Surprisingly, when different lengths of CsgF peptides that contained the same constriction region were reconstituted with the same CsgG pore to make CsgG:CsgF complexes, they gave signals with a different range (FIG. 24). CsgG:CsgF complex which contains the shortest CsgF peptide (1-29 of Seq ID No. 6) showed the largest range and the CsgG:CsgF complex which contains the longest CsgF peptide (1-45 of Seq ID No. 6) showed the smallest range (FIG. 24).

Materials and Methods for Characterisation of Analytes:

The proteins produced by the methods described below can be used interchangeably with those produced by the methods described above with respect to structural determination.

Methods Expression of the CsgG:CsgF or CsgG:FCP Complex by Co-Expression

Genes encoding the CsgG proteins and its mutants are constructed in the pT7 vector which contains ampicillin resistance gene. Genes encoding the CsgF or FCP proteins and its mutants are constructed in the pRham vector which contains Kanamycin resistant gene. 1 uL of both plasmids is mixed with 50 uL of Lemo(DE3)ΔCsgEFG for 10 minutes on ice. The sample is then heated at 42° C. for 45 seconds before being returned to ice for another 5 minutes. 150 uL of NEB SOC outgrowth medium is added and the sample is incubated at 37° C. with shaking at 250 rpm for 1 hour. The entire volume is spread onto an agar plate containing kanamycin (40 ug/mL), ampicillin (100 ug/mL) and chloramphenicol (34 ug/ml) and incubated overnight at 37° C. Single colony is taken from the plate and inoculated into 100 mL of LB media containing kanamycin (40 ug/mL), ampicillin (100 ug/mL) and chloramphenicol (34 ug/ml) and incubated overnight at 37° C. with shaking at 250 rpm. 25 mL of the starter culture is added to 500 mL of LB media containing 3 mM ATP, 15 mM MgSO₄, kanamycin (40 ug/mL), ampicillin (100 ug/mL) and chloramphenicol (34 ug/ml) and incubated overnight at 37° C. The culture was allowed to grow for 7 hours, at which point the OD₆₀₀ was greater than 3.0. Lactose (1.0% final concentration), glucose (0.2% final concentration) and rhamnose (2 mM final concentration) were added and the temperature dropped to 18° C. whist shaking is maintained at 250 rpm for 16 hours. Culture was centrifuged at 6000 rpm for 20 mins at 4° C. The supernatant was discarded and the pellet kept. Cells stored at −80° C. until purification.

Expression of the CsgG Pore with or without a C-Term Strep Tag and CsgF with or without a C Terminal Strep or His Tag

All genes encoding all the CsgG proteins and CsgF or FCP proteins are constructed in the pT7 vector which contains ampicillin resistance gene. Expression procedure is same as above except for Kanacmycin is being omitted in all medias and buffers.

Cell lysis (co expressed complex or individual CsgG/CsgF/FCP proteins)

The lysis buffer is made of 50 mM Tris, pH 8.0, 150 mM NaCl, 0.1% DDM, 1× Bugbuster Protein Extraction Reagent (Merck), 2.5 uL Benzonase Nuclease (stock ≥250 units/μL)/100 mL of lysis buffer and 1 tablet Sigma Protease inhibitor cocktail/100 mL of lysis buffer. 5× volume of lysis buffer is used to lyse 1× weight of harvested cells. Cells resuspended and left to spin at room temperature for 4 hours until a homogenous lysate is produced. Lysate is spun at 20,000 rpm for 35 minutes at 4° C. The supernatant is carefully extracted and filtered through a 0.2 uM Acrodisc syringe filter.

Strep Purification of the CsgG or CsgF/FCP Proteins or Co-Expressed Complex if the CsgG Contains a C-Term Strep Tag and CsgF or FCP Contains a C-Term His Tag

The filtered sample was then loaded onto a 5 mL StrepTrap column with the following parameters: Loading speed: 0.8 mL/min, Complete sample loading: 10 mL, Wash out unbound: 10 CV (5 mL/min), Extra wash: 10 CV (5 mL/min), Elution: 3 CV (5 mL/min). Affinity buffer: 50 mL Tris, pH 8.0, 150 mM NaCl, 0.1% DDM; Wash buffer: 50 mL Tris, pH 8.0, 2M NaCl, 0.1% DDM; Elution buffer: 50 mL Tris, pH8.0, 150 mM NaCl, 0.1% DDM, 10 mM desthiobiotin. Eluted sample is collected.

His Purification of the CsgG or CsgF/FCP Proteins or Co-Expressed Complex if the CsgG Contains a C-Term Strep Tag and CsgF or FCP Contains a C-Term His Tag

Filtered sample or pooled eluted peaks from Strep purification (in case of the complex) loaded onto 5 mL HisTrap column using the same parameters as above, except with the following buffers: Affinity & wash buffer: 50 mL Tris, pH 8.0, 150 mM NaCl, 0.1% DDM, 25 mM imidazole; Elution: 50 mL Tris, pH 8.0, 150 mM NaCl, 0.1% DDM, 350 mM imidazole. Peak eluted, concentrated in 30 kDa MWCO Merck Milipore centrifugal unit to a volume of 500 uL.

Formation of the Complex In Vitro with In Vivo Purified Components.

Both the CsgG and the CsgF/FCP proteins expressed and purified separately are mixed in various ratios to identify the correct ratio. however always in excess CsgF conditions. The complex was then incubated overnight at 25° C. To remove the excess CsgF and remove DTT from the buffer, the mixture was again injected onto the Superdex Increase 200 10/300 equilibrated in 50 mM Tris, pH 8.0, 150 mM NaCl, 0.1% DDM. The complex usually elutes between 9 to 10 mL on this column.

Polishing Step with Gel Filtration for the Complex (Co-Expressed or Made In Vitro) If necessary, Strep purified or His purified or His followed by Strep purified CsgG:CsgF or CsgG:FCP can be subjected to a further polishing step by gel filtration. 500 uL of the sample was injected into a 1 mL sample loop and onto the Superdex Increase 200 10/300 equilibrated in 50 mM Tris, pH 8.0, 150 mM NaCl, 0.1% DDM. The peak associated to the complex usually elutes between 9 and 10 mL on this column when run 1 mL/min. Sample was heated at 60° C. for 15 minutes and centrifuged at 21,000rcf for 10 mins. Supernatant was taken for testing. Samples were subjected to SDS-PAGE to confirm and identify fractions eluted with the complex.

Cleavage of CsgF or FCP at the TEV Protease Site

If the CsgF or FCP contains a TEV cleavage site, TEV-protease with a C-term Histidine tag is added to the sample (amount added is identified based on the rough concentration of the protein complex) with 2 mM DTT. Sample incubated overnight at 4° C. on the roller mixer at 25 rpm. The mixture is then run back through a 5 mL HisTrap column and the flow through is collected. Anything uncleaved will remain bound to the column and the cleaved protein will elute. Same buffers and parameters and the final heating step are used as in the His purification described above.

Purifying the CsgG:FCP Complex with In Vivo Purified CsgG Pore and Synthetic FCP

Lyophilised FCP peptides received from Genscript and Lifetein. 1 mg of peptide dissolved in 1 mL of nuclease free ddH₂O to obtain 1 mg/mL sample. Sample was vortexed until no peptide remains visible. Due to differences in expression levels of CsgG pores and mutants, it's difficult to measure the concentration accurately. Intensity of protein bands on SDS-PAGE against known markers can be used to get a rough estimate of the sample. CsgG and FCP are then mixed in approximately 1:50 molar ratio and incubate at 25° C. overnight at 700 rpm. Samples were heated at 60° C. for 15 minutes and centrifuged at 21,000rcf for 10 mins. Supernatant was taken for testing. If needed, the complex can be purified as detailed above in co-expression.

Purifying CsgG:CsgF or CsgG:FCP Containing Cysteine Mutants

Same procedure as above can be used to purify the CsgG:CsgF or CsgG:FCP complexes (with I or II or III below) if either or both components contain cysteines except for the composition of affinity, wash and elution buffers in His and Strep purifications and the buffer used in gel filtration. To purify cysteine mutants, all these buffers should contain 2 mM DTT. 2 mM DTT was also been added when synthetic peptides containing cysteines are dissolved in ddH2O

I. co-expression of CsgG and CsgF or FCP

II. Making the CsgG:CsgF or CsgG:FCP complexes in vitro with in vivo purified individual components

III. Making the CsgG:CsgF or CsgG:FCP complexes in vitro with in vivo purified CsgG and synthetic FCP

Determination of Cys-Bond Formation

Two tubes of 50 uL each from the final elution were separated. In one of the tube, 2 mM DTT was added as a reducing agent and in the other tube 100 μM of Cu(II):1-10 Phenanthroline (33 mM: 100 mM) was added as an oxidizing agent. Samples were mixed 1:1 with Laemmli buffer containing 4% SDS. Half the sample were heat treated to 100deg for 10 min (denaturating condition) and half of them were left untreated, before running on a 4-20% TGX gel (Bio-rad Criterion) in TGS buffer.

Coupled In Vitro Transcription and Translation (IVTT)

All proteins were generated by coupled in vitro transcription and transition (IVTT) by using an E. coli T7-S30 extract system for circular DNA (Promega). The complete 1 mM amino acid mixture minus cysteine and the complete 1 mM amino acid mixture minus methionine were mixed in equal volumes to obtain the working amino acid solution required to generate high concentrations of the proteins. The amino acids (10 uL) were mixed with premix solution (40 uL), [35S]L-methionine (2 uL, 1175 Ci/mmol, 10 mCi/mL), plasmid DNA (16 uL, 400 ng/uL) and T7 S30 extract (30 uL) and rifarnpicin (2 uL, 20 mg/mL) to generate a 100 uL reaction of IVTT proteins. Synthesis was carried out for 4 hours at 30° C. followed by overnight incubation at room temperature. If the CsgG:CsgF or CsgG; FCP complexes were made in co-expression, plasmid DNAs encoding each component were mixed in equal amounts, and a portion of the mixture (16 uL) was used for IVTT. After incubation, the tube was centrifuged for 10 minutes at 22000g, of which the supernatant was discarded. The resulting pellet was resuspended and washed in MBSA (10 mM MOPS, 1 mg/ml BSA pH7.4) and centrifuged again under the same conditions, The protein present in the pellet was re-suspended in 1× Laemmli sample buffer and run in 4-20% TGX gel at 300V for 25 min. The gel was then dried and exposed to Carestream® Kodak® BioMax® MR film overnight. The film was then processed and the protein in the gel visualized,

Samples for Testing in MinIONs

All samples prior to testing are incubated with Brij58 (final concentration of 0.1%) for 10 minutes at room temperature before making up subsequent pore dilutions necessary for pore insertion.

Method for Preparing and Running Static Strands

A set of polyA DNA strands (SS20 to SS38 of FIG. 27) in which one base is missing from the DNA backbone (iSpc3) is obtained by integrated DNA Technologies (iDT). 3′ end of each of these strand also comprise a biotin modification. The static strands are incubated with monovalent streptavidin at room temperature for 20 minutes, resulting in the biotin binding to the streptavidin. The streptavidin-static strand complex was diluted to 500 nM (B, FIG. 27) and 2 uM (C, FIG. 27) in 25 mM HEPES, 430 mM KCl, 30 mM ATP, 30 mM MgCl2, 2.15 mM EDTA, pH8 (known as RBFM). The residual current generated by each static strand is recorded in a MinION set up, MinION flow cells were flushed as per standard running protocols, and then the sequencing protocol was started with 1 minute static flicks, initially 10 minutes of open pore recording was generated before 150 uL of the first streptavidin-static strand complex was added. After 10 minutes, 300 uL of RBFM was flushed through the flow ceil before the next streptavidin-static strand complex was added. This process was repeated for ail streptavidin-static strands. Once the final streptavidin-static strand complex had been incubated on the flow cell, 800 uL of RBFM was flushed through the flow cell and 10 minutes of open pore recording was generated before finishing the experiment.

Method for Making Discrimination Profile Plots

The reader head discrimination profiles show the average variation in modelled current when the base at each reader head position is varied. To calculate the reader head discrimination at position i for a model of length k with alphabet of length n, we defined the discrimination at reader head position as the median of the standard deviations in current level for each of the n^(k) groups of size n where position i is varied while other positions are held constant.

Aspects of the Disclosure

1. An isolated pore complex comprising a CsgG pore, or a homologue or mutant thereof, and a modified CsgF peptide, or a homologue or mutant thereof. 2. The isolated pore complex according to 1, wherein the modified CsgF peptide, or a homologue or mutant thereof, is inserted into the lumen of the CsgG pore, or a homologue or mutant thereof. 3. The isolated pore complex according to 2, wherein the pore complex has two or more channel constrictions, comprising a CsgG channel constriction and a CsgF channel constriction. 4. The isolated pore complex according to any one of 1 to 3, wherein the CsgG pore, or homologue or mutant thereof, is a mutant CsgG pore. 5. The isolated pore complex according to 3 or 4, wherein the CsgF channel constriction has a diameter in the range from 0.5 nm to 2.0 nm 6. A modified CsgF peptide, or a modified peptide of a CsgF homologue or mutant, wherein the modification comprises a truncation of the CsgF protein SEQ ID NO:6 or of a homologue or mutant thereof. 7. A modified CsgF peptide, or a modified peptide of a CsgF homologue or mutant, according to 6, wherein said modified CsgF peptide comprises SEQ ID NO:39, or SEQ ID NO:40, or a homologue or mutant thereof. 8. A modified CsgF peptide, or a modified peptide of a CsgF homologue or mutant, according to 6, wherein said modified CsgF peptide comprises SEQ ID NO:15, or a homologue or mutant thereof. 9. A modified CsgF peptide according to 8, or a modified peptide of a CsgF homologue or mutant, wherein one or more positions in the region comprising SEQ ID NO:15 are mutated, with a minimal of 35% amino acid identity to SEQ ID NO:15. 10. A polynucleotide which encodes a modified CsgF peptide according to any one of 6 to 9. 11. The isolated pore complex according to any one of 1 to 5, wherein said modified CsgF peptide, or a homologue or mutant thereof, is a peptide according to any one of 6 to 9. 12. The isolated pore complex according to 11, wherein the modified CsgF peptide and the CsgG pore, or homologues or mutants thereof, are covalently coupled. 13. The isolated pore complex according to 12, wherein the covalent coupling is via: (i) a cysteine residue at a position corresponding to 132, 133, 136, 138, 140, 142, 144, 145, 147, 149, 151, 153, 155, 183, 185, 187, 189, 191, 201, 203, 205, 207 or 209 of SEQ ID NO: 3, or of a homologue thereof; (ii) a non-native reactive or photoreactive amino acid at a position corresponding to 132, 133, 136, 138, 140, 142, 144, 145, 147, 149, 151, 153, 155, 183, 185, 187, 189, 191, 201, 203, 205, 207 or 209 of SEQ ID NO: 3, or of a homologue thereof. 14. An isolated transmembrane pore complex comprising the isolated pore complex according to any one of 1 to 5 or 11 to 13, and the components of a membrane. 15. A method for producing a transmembrane pore complex, wherein the pore complex is formed by a CsgG pore and a modified CsgF peptide, or homologues or mutants thereof, comprising co-expressing CsgG as depicted in SEQ ID NO:2, or a homologue or mutant thereof, and a modified CsgF peptide, or a homologue or mutant thereof, in a suitable host cell, thereby allowing in vivo transmembrane pore complex formation. 16. The method according to 15, wherein the modified CsgF peptide, or homologue or mutant thereof, comprises SEQ ID NO:12 or SEQ ID NO:14, or a homologue or mutant thereof. 17. A method for producing an isolated pore complex, wherein the isolated pore is formed by a CsgG pore, or a homologue or mutant thereof, and a modified CsgF peptide, or a homologue or mutant thereof, comprising contacting the CsgG monomers of SEQ ID NO:3, or a homologue or mutant thereof, with modified CsgF peptide, or a homologue or mutant thereof, thereby allowing in vitro reconstitution of the isolated pore complex. 18. The method according to 17, wherein the modified CsgF peptide, or a homologue or mutant thereof, comprises SEQ ID NO:15 or SEQ ID NO:16, or a homologue or mutant thereof. 19. A method for determining the presence, absence or one or more characteristics of a target analyte, comprising the steps of: (i) contacting the target analyte with a pore complex according to any one of 1 to 5 or 11 to 13 or with a transmembrane pore complex according to 14, such that the target analyte moves into the pore complex; and (ii) taking one or more measurements as the analyte moves through the pore complex and thereby determining the presence, absence or one or more characteristics of the analyte. 20. A method according to 19, wherein the analyte is a polynucleotide. 21. A method according to 19, wherein the analyte is a (poly)peptide 22. A method according to 19, wherein the analyte is a polysaccharide 23. A method according to 19, wherein the analyte is a small organic or inorganic compound, such as pharmacologically active compounds, toxic compounds and pollutants. 24. A method according to 20, comprising determining one or more characteristics selected from (i) the length of the polynucleotide, (ii) the identity of the polynucleotide, (iii) the sequence of the polynucleotide, (iv) the secondary structure of the polynucleotide and (v) whether or not the polynucleotide is modified. 25. A method of characterising a polynucleotide or a (poly)peptide using an isolated transmembrane pore complex, wherein the pore complex is a complex comprising a CsgG pore, or a homologue or mutant thereof, and a modified CsgF peptide, or a homologue or mutant thereof. 26. A method according to 25, wherein the CsgG pore, or homologue or mutant thereof, comprises six to ten monomers. 27. Use of the isolated pore complex according to any one of 1 to 5 or 11 to 13 or of the transmembrane pore complex according to 14 to determine the presence, absence or one or more characteristics of a target analyte. 28. A kit for characterising a target analyte comprising (a) an isolated pore complex according to any one of 1 to 5 or 11 to 13 and (b) the components of a membrane.

Sequences Description of the Sequences:

SEQ ID NO:1 shows polynucleotide sequence of wild-type E. coli CsgG from strain K12, including signal sequence (Gene ID: 945619). SEQ ID NO:2 shows amino acid sequence of wild-type E. coli CsgG including signal sequence (Uniprot accession number P0AEA2). SEQ ID NO:3 shows amino acid sequence of wild-type E. coli CsgG as mature protein (Uniprot accession number P0AEA2). SEQ ID NO:4 shows polynucleotide sequence of wild-type E. coli CsgF from strain K12, including signal sequence (Gene ID: 945622). SEQ ID NO:5 shows amino acid sequence of wild-type E. coli CsgF including signal sequence (Uniprot accession number P0AE98). SEQ ID NO:6 shows amino acid sequence of wild-type E. coli CsgF as mature protein (Uniprot accession number P0AE98). SEQ ID NO:7 shows polynucleotide sequence of a fragment of wild-type E. coli CsgF encoding amino acids 1 to 27 and a C-terminal 6 His tag. SEQ ID NO:8 shows amino acid sequence of a fragment of wild-type E. coli CsgF encompassing amino acids 1 to 27 and a C-terminal 6 His tag. SEQ ID NO:9 shows polynucleotide sequence of a fragment of wild-type E. coli CsgF encoding amino acids 1 to 38 and a C-terminal 6 His tag. SEQ ID NO:10 shows amino acid sequence of a fragment of wild-type E. coli CsgF encompassing amino acids 1 to 38 and a C-terminal 6 His tag. SEQ ID NO:11 shows polynucleotide sequence of a fragment of wild-type E. coli CsgF encoding amino acids 1 to 48 and a C-terminal 6 His tag. SEQ ID NO:12 shows amino acid sequence of a fragment of wild-type E. coli CsgF encompassing amino acids 1 to 48 and a C-terminal 6 His tag. SEQ ID NO:13 shows polynucleotide sequence of a fragment of wild-type E. coli CsgF encoding amino acids 1 to 64 and a C-terminal 6 His tag. SEQ ID NO:14 shows amino acid sequence of a fragment of wild-type E. coli CsgF encompassing amino acids 1 to 64 and a C-terminal 6 His tag. SEQ ID NO:15 shows amino acid sequence of a peptide corresponding to residues 20 to 53 of E. coli CsgF SEQ ID NO:16 shows amino acid sequence of a peptide corresponding to residues 20 to 42 of E. coli CsgF, including KD at its C-terminus SEQ ID NO:17 shows amino acid sequence of a peptide corresponding to residues 23 to 55 of CsgF homologue Q88H88 SEQ ID NO:18 shows amino acid sequence of a peptide corresponding to residues 25 to 57 of CsgF homologue A0A143HJA0 SEQ ID NO:19 shows amino acid sequence of a peptide corresponding to residues 21 to 53 of CsgF homologue Q5E245 SEQ ID NO:20 shows amino acid sequence of a peptide corresponding to residues 19 to 51 of CsgF homologue Q084E5 SEQ ID NO:21 shows amino acid sequence of a peptide corresponding to residues 15 to 47 of CsgF homologue FOLZU2 SEQ ID NO:22 shows amino acid sequence of a peptide corresponding to residues 26 to 58 of CsgF homologue A0A136HQR0 SEQ ID NO:23 shows amino acid sequence of a peptide corresponding to residues 21 to 53 of CsgF homologue A0A0W1 SRL3 SEQ ID NO:24 shows amino acid sequence of a peptide corresponding to residues 26 to 59 of CsgF homologue B0UH01 SEQ ID NO:25 shows amino acid sequence of a peptide corresponding to residues 22 to 53 of CsgF homologue Q6NAU5 SEQ ID NO:26 shows amino acid sequence of a peptide corresponding to residues 7 to 38 of CsgF homologue G8PUY5 SEQ ID NO:27 shows amino acid sequence of a peptide corresponding to residues 25 to 57 of CsgF homologue A0A0S2ETP7 SEQ ID NO:28 shows amino acid sequence of a peptide corresponding to residues 19 to 51 of CsgF homologue E311Z1 SEQ ID NO:29 shows amino acid sequence of a peptide corresponding to residues 24 to 55 of CsgF homologue F3Z094 SEQ ID NO:30 shows amino acid sequence of a peptide corresponding to residues 21 to 53 of CsgF homologue A0A176T7M2 SEQ ID NO:31 shows amino acid sequence of a peptide corresponding to residues 14 to 45 of CsgF homologue D2QPP8 SEQ ID NO:32 shows amino acid sequence of a peptide corresponding to residues 28 to 58 of CsgF homologue N2IYT1 SEQ ID NO:33 shows amino acid sequence of a peptide corresponding to residues 26 to 58 of CsgF homologue W7QHV5 SEQ ID NO:34 shows amino acid sequence of a peptide corresponding to residues 23 to 55 of CsgF homologue D4ZLW2 SEQ ID NO:35 shows amino acid sequence of a peptide corresponding to residues 21 to 53 of CsgF homologue D2QT92 SEQ ID NO:36 shows amino acid sequence of a peptide corresponding to residues 20 to 51 of CsgF homologue A0A167UJA2 SEQ ID NO:37 shows amino acid sequence of a fragment of wild-type E. coli CsgF encompassing amino acids 20 to 27. SEQ ID NO:38 shows amino acid sequence of a fragment of wild-type E. coli CsgF encompassing amino acids 20 to 38. SEQ ID NO:39: shows amino acid sequence of a fragment of wild-type E. coli CsgF encompassing amino acids 20 to 48. SEQ ID NO:40 shows amino acid sequence of a fragment of wild-type E. coli CsgF encompassing amino acids 20 to 64. SEQ ID NO:41 shows the nucleotide sequence of primer CsgF_d27_end SEQ ID NO:42 shows the nucleotide sequence of primer CsgF_d38_end SEQ ID NO:43 shows the nucleotide sequence of primer CsgF_d48_end SEQ ID NO:44 shows the nucleotide sequence of primer CsgF_d64_end SEQ ID NO:45 shows the nucleotide sequence of primer pNa62_CsgF_histag_Fw SEQ ID NO:46 shows the nucleotide sequence of primer CsgF-His_pET22b_FW SEQ ID NO:47 shows the nucleotide sequence of primer CsgF-His_pET22b_Rev SEQ ID NO:48 shows the nucleotide sequence of primer csgEFG_pDONR221_FW SEQ ID NO:49 shows the nucleotide sequence of primer csgEFG_pDONR221_Rev SEQ ID NO:50 shows the nucleotide sequence of primer Mut_csgF_His_FW SEQ ID NO:51 shows the nucleotide sequence of primer Mut_csgF_His_Rev SEQ ID NO:52 shows the nucleotide sequence of primer DelCsgE_Rev SEQ ID NO:53 shows the nucleotide sequence of primer DelCsgE FW SEQ ID NO: 54 shows the amino acid sequence of residues 1 to 30 of mature E. coli CsgF SEQ ID NO: 55 shows the amino acid sequence of residues 1 to 35 of mature E. coli CsgF SEQ ID NO: 56 shows the amino acid sequence of a mutated (T4C/N17S) CsgF sequence with a signal sequence, and a TEV protease cleavage site (ENLYFQS) inserted between residues 35 and 36 of sequence of the mature protein. SEQ ID NO: 57 shows the amino acid sequence of a mutated (N17S-Del) CsgF sequence with a signal sequence, and a TEV protease cleavage site (ENLYFQS) inserted between residues 35 and 36 of sequence of the mature protein. SEQ ID NO: 58 shows the amino acid sequence of a mutated (G1C/N17S) CsgF sequence with a signal sequence, and a TEV protease cleavage site (ENLYFQS) inserted between residues 35 and 36 of sequence of the mature protein. SEQ ID NO: 59 shows the amino acid sequence of a mutated (G1C) CsgF sequence with a signal sequence, and a TEV protease cleavage site (ENLYFQS) inserted between residues 35 and 36 of sequence of the mature protein. SEQ ID NO: 60 shows the amino acid sequence of a CsgF sequence with a signal sequence, a TEV protease cleavage site (ENLYFQS) inserted between residues 45 and 46 of sequence of the mature protein, and a His₁₀ tag at the C-terminus. SEQ ID NO: 61 shows the amino acid sequence of a CsgF sequence with a signal sequence, a TEV protease cleavage site (ENLYFQS) inserted between residues 35 and 36 of sequence of the mature protein, and a His₁₀ tag at the C-terminus. SEQ ID NO: 62 shows the amino acid sequence of a CsgF sequence with a signal sequence, a TEV protease cleavage site (ENLYFQS) inserted between residues 30 and 31 of sequence of the mature protein, and a His₁₀ tag at the C-terminus. SEQ ID NO: 63 shows the amino acid sequence of a CsgF sequence with a signal sequence, a TEV protease cleavage site (ENLYFQS) inserted between residues 45 and 51 of sequence of the mature protein, and a His₁₀ tag at the C-terminus. SEQ ID NO: 64 shows the amino acid sequence of a CsgF sequence with a signal sequence, a TEV protease cleavage site (ENLYFQS) inserted between residues 30 and 37 of sequence of the mature protein, and a His₁₀ tag at the C-terminus. SEQ ID NO: 65 shows the amino acid sequence of a CsgF sequence with a signal sequence, a HCV C3 protease cleavage site (LEVLFQGP) inserted between residues 34 and 36 of sequence of the mature protein, and a His₁₀ tag at the C-terminus. SEQ ID NO: 66 shows the amino acid sequence of a CsgF sequence with a signal sequence, a HCV C3 protease cleavage site (LEVLFQGP) inserted between residues 42 and 43 of sequence of the mature protein, and a His₁₀ tag at the C-terminus. SEQ ID NO: 67 shows the amino acid sequence of a CsgF sequence with a signal sequence, a HCV C3 protease cleavage site (LEVLFQGP) inserted between residues 38 and 47 of sequence of the mature protein, and a His₁₀ tag at the C-terminus. SEQ ID NO: 68 shows the amino acid sequence of YP_001453594.1: 1-248 of hypothetical protein CKO_02032 [Citrobacter koseri ATCC BAA-895], which is 99% identical to SEQ ID NO: 3. SEQ ID NO: 69 shows the amino acid sequence of WP_001787128.1: 16-238 of curli production assembly/transport component CsgG, partial [Salmonella enterica], which is 98% to SEQ ID NO: 3. SEQ ID NO: 70 shows the amino acid sequence of KEY44978.1|:16-277 of curli production assembly/transport protein CsgG [Citrobacter amalonaticus], which is 98% identical to SEQ ID NO: 3. SEQ ID NO: 71 shows the amino acid sequence of YP_003364699.1: 16-277 of curli production assembly/transport component [Citrobacter rodentium ICC 68], which is 97% identical to SEQ ID NO: 3. SEQ ID NO: 72 shows the amino acid sequence of YP_004828099.1: 16-277 of curli production assembly/transport component CsgG [Enterobacter asburiae LF7a], which is 94% identical to SEQ ID NO: 3. SEQ ID NO: 73 shows the amino acid sequence of WP_006819418.1: 19-280 of transporter [Yokenella regensburgei], which is 91% identical to SEQ ID NO: 3. SEQ ID NO: 74 shows the amino acid sequence of WP_024556654.1: 16-277 of curli production assembly/transport protein CsgG [Cronobacter pulveris], which is 89% identical to SEQ ID NO: 3. SEQ ID NO: 75 shows the amino acid sequence of YP_005400916.1:16-277 of curli production assembly/transport protein CsgG [Rahnella aquatilis HX2], which is 84% identical to SEQ ID NO: 3. SEQ ID NO: 76 shows the amino acid sequence of KFC99297.1: 20-278 of CsgG family curli production assembly/transport component [Kluyvera ascorbata ATCC 33433], which is 82% identical to SEQ ID NO: 3. SEQ ID NO: 77 shows the amino acid sequence of KFC86716.11:16-274 of CsgG family curli production assembly/transport component [Hafnia alvei ATCC 13337], which is 81% identical to SEQ ID NO: 3. SEQ ID NO: 78 shows the amino acid sequence of YP_007340845.11:16-270 of uncharacterised protein involved in formation of curli polymers [Enterobacteriaceae bacterium strain FGI 57], which is 76% identical to SEQ ID NO: 3. SEQ ID NO: 79 shows the amino acid sequence of WP_010861740.1: 17-274 of curli production assembly/transport protein CsgG [Plesiomonas shigelloides], which is 70% identical to SEQ ID NO: 3. SEQ ID NO: 80 shows the amino acid sequence of YP_205788.1: 23-270 of curli production assembly/transport outer membrane lipoprotein component CsgG [Vibrio fischeri ES114], which is 60% identical to SEQ ID NO: 3. SEQ ID NO: 81 shows the amino acid sequence of WP_017023479.1: 23-270 of curli production assembly protein CsgG [Aliivibrio logei], which is 59% identical to SEQ ID NO: 3. SEQ ID NO: 82 shows the amino acid sequence of WP_007470398.1: 22-275 of Curli production assembly/transport component CsgG [Photobacterium sp. AK 5], which is 57% identical to SEQ ID NO: 3. SEQ ID NO: 83 shows the amino acid sequence of WP_021231638.1: 17-277 of curli production assembly protein CsgG [Aeromonas veronii], which is 56% identical to SEQ ID NO: 3. SEQ ID NO: 84 shows the amino acid sequence of WP_033538267.1: 27-265 of curli production assembly/transport protein CsgG [Shewanella sp. ECSMB14101], which is 56% identical to SEQ ID NO: 3. SEQ ID NO: 85 shows the amino acid sequence of WP_003247972.1: 30-262 of curli production assembly protein CsgG [Pseudomonas putida], which is 54% identical to SEQ ID NO: 3. SEQ ID NO: 86 shows the amino acid sequence of YP_003557438.1: 1-234 of curli production assembly/transport component CsgG [Shewanella violacea DSS12], which is 53% identical to SEQ ID NO: 3. SEQ ID NO: 87 shows the amino acid sequence of WP_027859066.1: 36-280 of curli production assembly/transport protein CsgG [Marinobacterium jannaschii], which is 53% identical to SEQ ID NO: 3. SEQ ID NO: 88 shows the amino acid sequence of CEJ70222.1: 29-262 of Curli production assembly/transport component CsgG [Chryseobacterium oranimense G311], which is 50% identical to SEQ ID NO: 3.

(>P0AEA2; coding sequence for WT CsgG from E. coli K12) SEQ ID NO: 1 ATGCAGCGCTTATTTCTTTTGGTTGCCGTCATGTTACTGAGCGGATGCTTAACCGCCCCGCCTAA AGAAGCCGCCAGACCGACATTAATGCCTCGTGCTCAGAGCTACAAAGATTTGACCCATCTGCCA GCGCCGACGGGTAAAATCTTTGTTTCGGTATACAACATTCAGGACGAAACCGGGCAATTTAAACC CTACCCGGCAAGTAACTTCTCCACTGCTGTTCCGCAAAGCGCCACGGCAATGCTGGTCACGGCA CTGAAAGATTCTCGCTGGTTTATACCGCTGGAGCGCCAGGGCTTACAAAACCTGCTTAACGAGC GCAAGATTATTCGTGCGGCACAAGAAAACGGCACGGTTGCCATTAATAACCGAATCCCGCTGCA ATCTTTAACGGCGGCAAATATCATGGTTGAAGGTTCGATTATCGGTTATGAAAGCAACGTCAAAT CTGGCGGGGTTGGGGCAAGATATTTTGGCATCGGTGCCGACACGCAATACCAGCTCGATCAGAT TGCCGTGAACCTGCGCGTCGTCAATGTGAGTACCGGCGAGATCCTTTCTTCGGTGAACACCAGT AAGACGATACTTTCCTATGAAGTTCAGGCCGGGGTTTTCCGCTTTATTGACTACCAGCGCTTGCT TGAAGGGGAAGTGGGTTACACCTCGAACGAACCTGTTATGCTGTGCCTGATGTCGGCTATCGAA ACAGGGGTCATTTTCCTGATTAATGATGGTATCGACCGTGGTCTGTGGGATTTGCAAAATAAAGC AGAACGGCAGAATGACATTCTGGTGAAATACCGCCATATGTCGGTTCCACCGGAATCCTGA (>P0AEA2 (1:277); WT prepro CsgG from E. coli K12) SEQ ID NO: 2 MQRLFLLVAVMLLSGCLTAPPKEAARPTLMPRAQSYKDLTHLPAPTGKIFVSVYNIQDETGQFKPYPA SNFSTAVPQSATAMLVTALKDSRWFIPLERQGLQNLLNERKIIRAAQENGTVAINNRIPLQSLTAANIM VEGSIIGYESNVKSGGVGARYFGIGADTQYQLDQIAVNLRVVNVSTGEILSSVNTSKTILSYEVQAGVF RFIDYQRLLEGEVGYTSNEPVMLCLMSAIETGVIFLINDGIDRGLWDLQNKAERQNDILVKYRHMSVPP ES (>P0AEA2 (16:277); mature CsgG from E. coli K12) SEQ ID NO: 3 CLTAPPKEAARPTLMPRAQSYKDLTHLPAPTGKIFVSVYNIQDETGQFKPYPASNFSTAVPQSATAML VTALKDSRWFIPLERQGLQNLLNERKIIRAAQENGTVAINNRIPLQSLTAANIMVEGSIIGYESNVKSGG VGARYFGIGADTQYQLDQIAVNLRVVNVSTGEILSSVNTSKTILSYEVQAGVFRFIDYQRLLEGEVGYT SNEPVMLCLMSAIETGVIFLINDGIDRGLWDLQNKAERQNDILVKYRHMSVPPES (>P0AE98; coding sequence for WT CsgF from E. coli K12) SEQ ID NO: 4 ATGCGTGTCAAACATGCAGTAGTTCTACTCATGCTTATTTCGCCATTAAGTTGGGCTGGAACCAT GACTTTCCAGTTCCGTAATCCAAACTTTGGTGGTAACCCAAATAATGGCGCTTTTTTATTAAATAG CGCTCAGGCCCAAAACTCTTATAAAGATCCGAGCTATAACGATGACTTTGGTATTGAAACACCCT CAGCGTTAGATAACTTTACTCAGGCCATCCAGTCACAAATTTTAGGTGGGCTACTGTCGAATATTA ATACCGGTAAACCGGGCCGCATGGTGACCAACGATTATATTGTCGATATTGCCAACCGCGATGG TCAATTGCAGTTGAACGTGACAGATCGTAAAACCGGACAAACCTCGACCATCCAGGTTTCGGGTT TACAAAATAACTCAACCGATTTT (>P0AE98 (1:138); WT pre CsgF from E. coli K12) SEQ ID NO: 5 MRVKHAVVLLMLISPLSWAGTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKDPSYNDDFGIETPS ALDNFTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQVSGLQNN STDF (>P0AE98 (20:138); WT mature CsgF from E. coli K12) SEQ ID NO: 6 GTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKDPSYNDDFGIETPSALDNFTQAIQSQILGGLLSNI NTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQVSGLQNNSTDF (>P0AE98; coding sequence for CsgF 1:27_6His) SEQ ID NO: 7 ATGCGTGTCAAACATGCAGTAGTTCTACTCATGCTTATTTCGCCATTAAGTTGGGCTGGAACCAT GACTTTCCAGTTCCGTCATCACCATCACCATCACTAAGCCC (>P0AE98 (1:28); preprotein of CsgF 20:27_6His) SEQ ID NO: 8 MRVKHAVVLLMLISPLSWA GTMTFQFRHHHHHH (>P0AE98; coding sequence for CsgF 1:38_6His) SEQ ID NO: 9 ATGCGTGTCAAACATGCAGTAGTTCTACTCATGCTTATTTCGCCATTAAGTTGGGCTGGAACCAT GACTTTCCAGTTCCGTAATCCAAACTTTGGTGGTAACCCAAATAATGGCCATCACCATCACCATC ACTAAGCCC (>P0AE98 (1:39); preprotein of CsgF 20:38_6His) SEQ ID NO: 10 MRVKHAVVLLMLISPLSWAGTMTFQFRNPNFGGNPNNG HHHHHH (>P0AE98; coding sequence for CsgF 1:48_6His) SEQ ID NO: 11 ATGCGTGTCAAACATGCAGTAGTTCTACTCATGCTTATTTCGCCATTAAGTTGGGCTGGAACCAT GACTTTCCAGTTCCGTAATCCAAACTTTGGTGGTAACCCAAATAATGGCGCTTTTTTATTAAATAG CGCTCAGGCCCAACATCACCATCACCATCACTAAGCCC (>P0AE98 (1:49); preprotein of CsgF 20:48_6His) SEQ ID NO: 12 MRVKHAVVLLMLISPLSWAGTMTFQFRNPNFGGNPNNGAFLLNSAQAQ HHHHHH (>P0AE98; coding sequence for CsgF 1:64_6His) SEQ ID NO: 13 ATGCGTGTCAAACATGCAGTAGTTCTACTCATGCTTATTTCGCCATTAAGTTGGGCTGGAACCAT GACTTTCCAGTTCCGTAATCCAAACTTTGGTGGTAACCCAAATAATGGCGCTTTTTTATTAAATAG CGCTCAGGCCCAAAACTCTTATAAAGATCCGAGCTATAACGATGACTTTGGTATTGAAACA CATCACCATCACCATCACTAAGCCC (>P0AE98 (1:65); preprotein of CsgF 20:64_6His) SEQ ID NO: 14 MRVKHAVVLLMLISPLSWAGTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKDPSYNDDFGIETHH HHHH (>P0AE98 (20:53); mature peptide of CsgF 20:53) SEQ ID NO: 15 GTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKD (>P0AE98 (20:42); mature peptide of CsgF 20:42 + KD) SEQ ID NO: 16 GTMTFQFRNPNFGGNPNNGAFLLKD (>Q88H88_PSEPK (23:55)) SEQ ID NO: 17 TELVYTPVNPAFGGNPLNGTWLLNNAQAQNDY (>A0A143HJA0_9GAMM (25:57)) SEQ ID NO: 18 TELIYEPVNPNFGGNPLNGSYLLNNAQAQDRH (>Q5E245_VIBF1 (21:53)) SEQ ID NO: 19 SELVYTPVNPNFGGNPLNTSHLFGGANAINDY (>Q084E5_SHEFN (19:51)) SEQ ID NO: 20 TQLVYTPVNPAFGGSYLNGSYLLANASAQNEH (>F0LZU2_VIBFN (15:47)) SEQ ID NO: 21 SSLVYEPVNPTFGGNPLNTTHLFSRAEAINDY (>A0A136HQR0_9ALTE (26:58)) SEQ ID NO: 22 TELVYEPINPSFGGNPLNGSFLLSKANSQNAH (>A0A0W1SRL3_9GAMM (21:53)) SEQ ID NO: 23 TEIVYQPINPSFGGNPMNGSFLLQKAQSQNAH (>B0UH01_METS4 (26:59)) SEQ ID NO: 24 SSLVYQPVNPAFGGPQLNGSWLQAEANAQNIPQ (>Q6NAU5_RHOPA (22:53)) SEQ ID NO: 25 GSLVYTPTNPAFGGSPLNGSWQMQQATAGNH (>G8PUY5_PSEUV (7:38)) SEQ ID NO: 26 QQLIYQPTNPSFGGYAANTTHLFATANAQKTA (>A0A0S2ETP7_9RHIZ (25:57)) SEQ ID NO: 27 GDLVYTPVNPSFGGSPLNSAHLLSIAGAQKNA (>E3I1Z1_RHOVT (19:51)) SEQ ID NO: 28 AELGYTPVNPSFGGSPLNGSTLLSEASAQKPN (>F3Z094_DESAF (24:55)) SEQ ID NO: 29 TELVFSFTNPSFGGDPMIGNFLLNKADSQKR (>A0A176T7M2_9FLAO (21:53)) SEQ ID NO: 30 QQLVYKSINPFFGGGDSFAYQQLLASANAQND (>D2QPP8_SPILD (14:45)) SEQ ID NO: 31 QALVYHPNNPAFGGNTFNYQWMLSSAQAQDR (>N2IYT1_9PSED (26:58)) SEQ ID NO: 32 TELVYTPKNPAFGGSPLNGSYLLGNAQAQNDY (>W7QHV5_9GAMM (26:58)) SEQ ID NO: 33 GQLIYQPINPSFGGDPLLGNHLLNKAQAQDTK (>D4ZLW2_SHEVD (23:55)) SEQ ID NO: 34 TQLIYTPVNPNFGGSYLNGSYLLANASVQNDH (>D2QT92_SPILD (21:53)) SEQ ID NO: 35 QAFVYHPNNPNFGGNTFNYSWMLSSAQAQDRT (>A0A167UJA2_9FLAO (20:51)) SEQ ID NO: 36 QGLIYKPKNPAFGGDTFNYQWLASSAESQNK (>P0AE98 (20:28); mature peptide of CsgF 20:27) SEQ ID NO: 37 GTMTFQFR (>P0AE98 (20:39); mature peptide of CsgF 20:38) SEQ ID NO: 38 GTMTFQFRNPNFGGNPNNG (>P0AE98 (20:49); mature peptide of CsgF 20:48) SEQ ID NO: 39 GTMTFQFRNPNFGGNPNNGAFLLNSAQAQ (>P0AE98 (20:65); mature peptide of CsgF 20:64) SEQ ID NO: 40 GTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKDPSYNDDFGIET (CsgF_d27_end) SEQ ID NO: 41 ACGGAACTGGAAAGTCATGGTTCC (CsgF_d38_end) SEQ ID NO: 42 GCCATTATTTGGGTTACCACCAAAGTTTGG (CsgF_d48_end) SEQ ID NO: 43 TTGGGCCTGAGCGCTATTTAATAAAAAAGC (CsgF_d64_end) SEQ ID NO: 44 TGTTTCAATACCAAAGTCATCGTTATAGCTCGG (pNa62_CsgF_histag_Fw) SEQ ID NO: 45 CATCACCATCACCATCACTAAGCCC (CsgF-His_pET22b_FW) SEQ ID NO: 46 CCCCCATATGGGAACCATGACTTTCCAGTTCC SEQ ID NO: 47: (CsgF-His_pET22b_Rev) CCCCGAATTCCTAATGGTGATGGTGATGGTGGTAAAAATCGGTTGAGTTATTTTG SEQ ID NO: 48: (csgEFG_pDONR221_FW) GGGGACAAGTTTGTACAAAAAAGCAGGCTACCTCAGGCGATAAAGCCATGAAACGTTA SEQ ID NO: 49: (csgEFG_pDONR221_Rev) GGGGACCACTTTGTACAAGAAAGCTGGGTGTTTAAACTCATTTTTCGAACTGCGGGTGGCTCCAA GCGCTGG SEQ ID NO: 50: (Mut_csgF_His_FW) CAAAATAACTCAACCGATTTTCATCACCATCACCATCACTAAGCCCCAGCTTCATAAGG SEQ ID NO: 51: (Mut_csgF_His_Rev) CCTTATGAAGCTGGGGCTTAGTGATGGTGATGGTGATGAAAATCGGTTGAGTTATTTTG SEQ ID NO: 52: (DelCsgE_Rev) AGCCTGCTTTTTTGTACAAAC SEQ ID NO: 53: (DelCsgE FW) ATAAAAAATTGTTCGGAGGCTGC (>P0AE98 (20:50); mature peptide of CsgF 1:30) SEQ ID NO: 54 GTMTFQFRNPNFGGNPNNGAFLLNSAQAQN (>P0AE98 (20:54); mature peptide of CsgF 1:35) SEQ ID NO: 55 GTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKDP Examples of CsgF sequences with protease cleavage sites made into proteins. Signal peptide is shown in bold TEV protease cleavage site in bold and underline and HCV C3 protease cleavage site in underline. StrepII indicate the Strep tag at the C terminus, H10 indicates the 10×Histidine tag at the C terminus and ** indicates STOP codons.

Pro-CsgF-Eco-(WT-T4C/N17S/P35-TEV-S36)-StrepII SEQ ID NO: 56 MRVKHAVVLLMLISPLSWAGTMCFQFRNPNFGGNPSNGAFLLNSAQAQNSYKDP ENLYFQS SYND DFGIETPSALDNFTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQ VSGLQNNSTDFSAWSHPQFEK** Pro-CsgF-Eco-(WT-N17S-Del(P35-[TEV]-S36)-StrepII SEQ ID NO: 57 MRVKHAVVLLMLISPLSWAGTMTFURNPNFGGNPSNGAFLLNSAQAQNSYKDP ENLYFQS SYND DFGIETPSALDNFTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQ VSGLQNNSTDFSAWSHPQFEK** Pro-CsgF-Eco-(WT-G1C/N17S/P35-[TEV]-S36)-StrepII SEQ ID NO: 58 MRVKHAVVLLMLISPLSWACTMTFURNPNFGGNPSNGAFLLNSAQAQNSYKDP ENLYFQS SYND DFGIETPSALDNFTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQ VSGLQNNSTDFSAWSHPQFEK** Pro-CsgF-Eco-(WT-G1C/P35-[TEV]-S36)-StrepII SEQ ID NO: 59 MRVKHAVVLLMLISPLSWACTMTFURNPNFGGNPNNGAFLLNSAQAQNSYKDP ENLYFQS SYND DFGIETPSALDNFTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQ VSGLQNNSTDFSAWSHPQFEK** Pro-CsgF-Eco-(WT-T45-TEV-P46)-H10 SEQ ID NO: 60 MRVKHAVVLLMLISPLSWAGTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKDPSYNDDFGIET EN LYFQS PSALDNFTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQV SGLQNNSTDFHHHHHHHHHH** Pro-CsgF-Eco-(WT-P35-TEV-S36)-H10 SEQ ID NO: 61 MRVKHAVVLLMLISPLSWAGTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKDP ENLYFQS SYND DFGIETPSALDNFTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQ VSGLQNNSTDFHHHHHHHHHH** Pro-CsgF-Eco-(WT-N30-TEV-S31)-H10 SEQ ID NO: 62 MRVKHAVVLLMLISPLSWAGTMTFQFRNPNFGGNPNNGAFLLNSAQAQN ENLYFQS SYKDPSYND DFGIETPSALDNFTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQ VSGLQNNSTDFHHHHHHHHHH** Pro-CsgF-Eco-(WT-T45-TEV-F51)-H10 SEQ ID NO: 63 MRVKHAVVLLMLISPLSWAGTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKDPSYNDDFGIET EN LYFQS FTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQVSGLQN NSTDFHHHHHHHHHH** Pro-CsgF-Eco-(WT-N30-TEV-Y37)-H10 SEQ ID NO: 64 MRVKHAVVLLMLISPLSWAGTMTFQFRNPNFGGNPNNGAFLLNSAQAQN ENLYFQS YNDDFGIETP SALDNFTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQVSGLQN NSTDFHHHHHHHHHH** Pro-CsgF-Eco-(WT-D34-[C3]-S36) SEQ ID NO: 65 MRVKHAVVLLMLISPLSWACTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKDLEVLFQGPSYND DFGIETPSALDNFTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQ VSGLQNNSTDFSAWSHPQFEK** Pro-CsgF-Eco-(WT-I42-[C3]-E43) SEQ ID NO: 66 MRVKHAVVLLMLISPLSWACTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKDPSYNDDFGILEVL FQGPETPSALDNFTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQ VSGLQNNSTDFSAWSHPQFEK** Pro-CsgF-Eco-(WT-N38-[C3]-S47) SEQ ID NO: 67 MRVKHAVVLLMLISPLSWACTMTFQFRNPNFGGNPNNGAFLLNSAQAQNSYKDPSYNLEVLFQGPS ALDNFTQAIQSQILGGLLSNINTGKPGRMVTNDYIVDIANRDGQLQLNVTDRKTGQTSTIQVSGLQNN STDFSAWSHPQFEK** SEQ ID NO: 68 MPRAQSYKDLTHLPMPTGKIFVSVYNIQDETGQFKPYPASNFSTAVPQSATAMLVTALKDSRWFIPLE RQGLQNLLNERKIIRAAQENGTVAINNRIPLQSLTAANIMVEGSIIGYESNVKSGGVGARYFGIGADT QYQLDQIAVNLRVVNVSTGEILSSVNTSKTILSYEVQAGVFRFIDYQRLLEGEIGYTSNEPVMLCLMS AIETGVIFLINDGIDRGLWDLQNKAERQNDILVKYRHMSVPPES SEQ ID NO: 69 CLTAPPKQAAKPTLMPRAQSYKDLTHLPAPTGKIFVSVYNIQDETGQFKPYPASNFSTAVPQSATAML VTALKDSRWFIPLERQGLQNLLNERKIIRAAQENGTVAMNNRIPLQSLTAANIMVEGSIIGYESNVKS GGVGARYFGIGADTQYQLDQIAVNLRVVNVSTGEILSSVNTSKTILSYEVQAGVFRFIDYQRLLEGEI GYTSNEPVMLCLMSAIETG SEQ ID NO: 70 CLTAPPKEAAKPTLMPRAQSYKDLTHLPIPTGKIFVSVYNIQDETGQFKPYPASNFSTAVPQSATAML VTALKDSRWFVPLERQGLQNLLNERKIIRAAQENGTVAINNRIPLQSLTAANIMVEGSIIGYESNVKS GGVGARYFGIGADTQYQLDQIAVNLRVVNVSTGEILSSVNTSKTILSYEVQAGVFRFIDYQRLLEGEI GYTSNEPVMLCLMSAIETGVIFLINDGIDRGLWDLQNKADRQNDILVKYRHMSVPPES SEQ ID NO: 71 CLTTPPKEAAKPTLMPRAQSYKDLTHLPVPTGKIFVSVYNIQDETGQFKPYPASNFSTAVPQSATAML VTALKDSRWFIPLERQGLQNLLNERKIIRAAQENGTVAINNRIPLPSLTAANIMVEGSIIGYESNVKS GGAGARYFGIGADTQYQLDQIAVNLRVVNVSTGEILSSVNTSKTILSYEVQAGVFRFIDYQRLLEGEI GYTSNEPVMLCLMSAIETGVIFLINDGIDRGLWDLQNKADRQNDILVKYRQMSVPPES SEQ ID NO: 72 CLTAPPKEAAKPTLMPRAQSYRDLTHLPAPTGKIFVSVYNIQDETGQFKPYPASNFSTAVPQSATAML VTALKDSHWFIPLERQGLQNLLNERKIIRAAQENGTVANNNRMPLQSLAAANVMIEGSIIGYESNVKS GGVGARYFGIGADTQYQLDQIAVNLRVVNVSTGEVLSSVNTSKTILSYEVQAGVFRFIDYQRLLEGEI GYTSNEPVMMCLMSAIETGVIFLINDGIDRGLWDLQNKADAQNPVLVKYRDMSVPPES SEQ ID NO: 73 CLTAPPKEAAKPTLMPRAQSYRDLTHLPLPSGKVFVSVYNIQDETGQFKPYPASNFSTAVPQSATAMLVTALKDS RWFVPLERQGLQNLLNERKIIRAAQENGTVADNNRIPLQSLTAANVMIEGSIIGYESNVKSGGVGARYFGIGADT QYQLDQIAVNLRVVNVSTGEVLSSVNTSKTILSYEVQAGVFRFVDYQRLLEGEIGYTSNEPVMLCLMSAIETGVI YLINDGIERGLWDLQQKADVDNPILARYRNMSAPPES SEQ ID NO: 74 CLTAPPKEAAKPTLMPRAQSYRDLTNLPDPKGKLFVSVYNIQDETGQFKPYPASNFSTAVPQSATSMLVTALKDS RWFIPLERQGLQNLLNERKIIRAAQENGTVAENNRMPLQSLVAANVMIEGSIIGYESNVKSGGVGARYFGIGGDT QYQLDQIAVNLRVVNVSTGEVLSSVNTSKTILSYEVQAGVFRFIDYQRLLEGEIGYTANEPVMLCLMSAIETGVI HLINDGINRGLWELKNKGDAKNTILAKYRSMAVPPES SEQ ID NO: 75 CLTAAPKEAARPTLLPRAPSYTDLTHLPSPQGRIFVSVYNIQDETGQFKPYPACNFSTAVPQSATAMLVSALKDS KWFIPLERQGLQNLLNERKIIRAAQENGSVAINNQRPLSSLVAANILIEGSIIGYESNVKSGGVGARYFGIGAST QYQLDQIAVNLRAVDVNTGEVLSSVNTSKTILSYEVQAGVFRFIDYQRLLEGELGYTTNEPVMLCLMSAIESGVI YLVNDGIERNLWQLQNPSEINSPILQRYKNNIVPAES SEQ ID NO: 76 CITSPPKQAAKPTLLPRSQSYQDLTHLPEPQGRLFVSVYNISDETGQFKPYPASNFSTSVPQSATAMLVSALKDS NWFIPLERQGLQNLLNERKIIRAAQENGTVAVNNRTQLPSLVAANILIEGSIIGYESNVKSGGAGARYFGIGAST QYQLDQIAVNLRVVNVSTGEVLSSVNTSKTILSYEFQAGVFRYIDYQRLLEGEVGYTVNEPVMLCLMSAIETGVI YLVNDGISRNLWQLKNASDINSPVLEKYKSIIVP SEQ ID NO: 77 CLTAPPKQAAKPTLMPRAQSYQDLTHLPEPAGKLFVSVYNIQDETGQFKPYPASNFSTAVPQSATAMLVSALKDS GWFIPLERQGLQNLLNERKIIRAAQENGTAAVNNQHQLSSLVAANVLVEGSIIGYESNVKSGGAGARFFGIGAST QYQLDQIAVNLRVVDVNTGQVLSSVNTSKTILSYEVQAGVFRYIDYQRLLEGEIGYTTNEPVMLCVMSAIETGVI YLVNDGINRNLWTLKNPQDAKSSVLERYKSTIVP SEQ ID NO: 78 CITTPPQEAAKPTLLPRDATYKDLVSLPQPRGKIYVAVYNIQDETGQFQPYPASNFSTSVPQSATAML VSSLKDSRWFVPLERQGLNNLLNERKIIRAAQQNGTVGDNNASPLPSLYSANVIVEGSIIGYASNVKT GGFGARYFGIGGSTQYQLDQVAVNLRIVNVHTGEVLSSVNTSKTILSYEIQAGVFRFIDYQRLLEGEA GFTTNEPVMTCLMSAIEEGVIHLINDGINKKLWALSNAADINSEVLTRYRK SEQ ID NO: 79 ITEVPKEAAKPTLMPRASTYKDLVALPKPNGKIIVSVYSVQDETGQFKPLPASNFSTAVPQSGNAMLTSALKDSG WFVPLEREGLQNLLNERKIIRAAQENGTVAANNQQPLPSLLSANVVIEGAIIGYDSDIKTGGAGARYFGIGADGK YRVDQVAVNLRAVDVRTGEVLLSVNTSKTILSSELSAGVFRFIEYQRLLELEAGYTTNEPVMMCMMSALEAGVAH LIVEGIRQNLWSLQNPSDINNPIIQRYMKEDVP SEQ ID NO: 80 PETSESPTLMQRGANYIDLISLPKPQGKIFVSVYDFRDQTGQYKPQPNSNFSTAVPQGGTALLTMALLDSEWFYP LERQGLQNLLTERKIIRAAQKKQESISNHGSTLPSLLSANVMIEGGIVAYDSNIKTGGAGARYLGIGGSGQYRAD QVTVNIRAVDVRSGKILTSVTTSKTILSYEVSAGAFRFVDYKELLEVELGYTNNEPVNIALMSAIDSAVIHLIVK GVQQGLWRPANLDTRNNPIFKKY SEQ ID NO: 81 PDASESPTLMQRGATYLDLISLPKPQGKIYVSVYDFRDQTGQYKPQPNSNFSTAVPQGGTALLTMALLDSEWFYP LERQGLQNLLTERKIIRAAQKKQESISNHGSTLPSLLSANVMIEGGIVAYDSNIKTGGAGARYLGIGGSGQYRAD QVTVNIRAVDVRSGKILTSVTTSKTILSYELSAGAFRFVDYKELLEVELGYTNNEPVNIALMSAIDSAVIHLIVK GIEEGLWRPENQNGKENPIFRKY SEQ ID NO: 82 PETSKEPTLMARGTAYQDLVSLPLPKGKVYVSVYDFRDQTGQYKPQPNSNFSTAVPQGGAALLTTALLDSRWFMP LEREGLQNLLTERKIIRAAQKKDEIPTNHGVHLPSLASANIMVEGGIVAYDTNIQTGGAGARYLGVGASGQYRTD QVTVNIRAVDVRTGRILLSVTTSKTILSKELQTGVFKFVDYKDLLEAELGYTTNEPVNLAVMSAIDAAVVHVIVD GIKTGLWEPLRGEDLQHPIIQEYMNRSKP SEQ ID NO: 83 CATHIGSPVADEKATLMPRSVSYKELISLPKPKGKIVAAVYDFRDQTGQYLPAPASNFSTAVTQGGVAMLSTALW DSQWFVPLEREGLQNLLTERKIVRAAQNKPNVPGNNANQLPSLVAANILIEGGIVAYDSNVRTGGAGAKYFGIGA SGEYRVDQVTVNLRAVDIRSGRILNSVTTSKTVMSQQVQAGVFRFVEYKRLLEAEAGFSTNEPVQMCVMSAIESG VIRLIANGVRDNLWQLADQRDIDNPILQEYLQDNAP SEQ ID NO: 84 ASSSLMPKGESYYDLINLPAPQGVMLAAVYDFRDQTGQYKPIPSSNFSTAVPQSGTAFLAQALNDSSWFIPVERE GLQNLLTERKIVRAGLKGDANKLPQLNSAQILMEGGIVAYDTNVRTGGAGARYLGIGAATQFRVDTVTVNLRAVD IRTGRLLSSVTTTKSILSKEITAGVFKFIDAQELLESELGYTSNEPVSLCVASAIESAVVHMIADGIWKGAWNLA DQASGLRSPVLQKY SEQ ID NO: 85 QDSETPTLTPRASTYYDLINMPRPKGRLMAVVYGFRDQTGQYKPTPASSFSTSVTQGAASMLMDALSASGWFVVL EREGLQNLLTERKIIRASQKKPDVAENIMGELPPLQAANLMLEGGIIAYDTNVRSGGEGARYLGIDISREYRVDQ VTVNLRAVDVRTGQVLANVMTSKTIYSVGRSAGVFKFIEFKKLLEAEVGYTTNEPAQLCVLSAIESAVGHLLAQG IEQRLWQV SEQ ID NO: 86 MPKSDTYYDLIGLPHPQGSMLAAVYDFRDQTGQYKAIPSSNFSTAVPQSGTAFLAQALNDSSWFVPVE REGLQNLLTERKIVRAGLKGEANQLPQLSSAQILMEGGIVAYDTNIKTGGAGARYLGIGVNSKFRVDT VTVNLRAVDIRTGRLLSSVTTTKSILSKEVSAGVFKFIDAQDLLESELGYTSNEPVSLCVAQAIESAV VHMIADGIWKRAWNLADTASGLNNPVLQKY SEQ ID NO: 87 LTRRMSTYQDLIDMPAPRGKIVTAVYSFRDQSGQYKPAPSSSFSTAVTQGAAAMLVNVLNDSGWFIPLEREGLQN ILTERKIIRAALKKDNVPVNNSAGLPSLLAANIMLEGGIVGYDSNIHTGGAGARYFGIGASEKYRVDEVTVNLRA IDIRTGRILHSVLTSKKILSREIRSDVYRFIEFKHLLEMEAGITTNDPAQLCVLSAIESAVAHLIVDGVIKKSWS LADPNELNSPVIQAYQQQRI SEQ ID NO: 88 PSDPERSTMGELTPSTAELRNLPLPNEKIVIGVYKFRDQTGQYKPSENGNNWSTAVPQGTTTILIKALEDSRWFI PIERENIANLLNERQIIRSTRQEYMKDADKNSQSLPPLLYAGILLEGGVISYDSNTMTGGFGARYFGIGASTQYR QDRITIYLRAVSTLNGEILKTVYTSKTILSTSVNGSFFRYIDTERLLEAEVGLTQNEPVQLAVTEAIEKAVRSLI IEGTRDKIW

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1-61. (canceled)
 62. An apparatus for characterising a target analyte, the apparatus comprising; (a) an isolated pore complex comprising: (i) a modified CsgG pore, wherein the CsgG constriction loop of the CsgG pore comprises one or more deletions; and (ii) a modified CsgF peptide, wherein the modified CsgF peptide is bound to and forms a constriction in the modified CsgG pore; and (b) an in vitro membrane; wherein the isolated pore complex is inserted into the in vitro membrane.
 63. An apparatus according to claim 62, wherein the CsgG constriction loop spans residues 46 to 61 of SEQ ID NO:
 3. 64. An apparatus according to claim 62, wherein the CsgG constriction loop comprises a deletion at one or more of the following positions F48, K49, P50, Y51, P52, A53, S54, N55, F56 and
 557. 65. An isolated pore complex comprising: (i) a modified CsgG pore, wherein the CsgG constriction loop of the CsgG pore comprises one or more deletions; and (ii) a modified CsgF peptide, wherein the modified CsgF peptide is bound to and forms a constriction in the modified CsgG pore.
 66. An isolated pore complex according to claim 65, wherein the CsgF peptide is truncated and lacks the C-terminal head domain of CsgF and at least part of the neck domain of CsgF.
 67. An isolated pore complex according to claim 65, wherein the CsgG constriction loop spans residues 46 to 61 of SEQ ID NO:3
 68. An isolated pore complex according to claim 65, wherein the CsgG constriction loop comprises a deletion at one or more of the following positions F48, K49, P50, Y51, P52, A53, S54, N55, F56 and
 557. 69. A method for determining the presence, absence or one or more characteristics of a target analyte, comprising the steps of: (i) contacting the target analyte with an apparatus according to claim 62, such that the target analyte moves into the pore complex; and (ii) taking one or more measurements as the analyte moves through the pore complex and thereby determining the presence, absence or one or more characteristics of the analyte.
 70. A method according to claim 69, wherein the analyte is a peptide, polypeptide, polysaccharide, small organic compound, or small inorganic compound.
 71. A method according to claim 69, wherein analyte is a pharmacologically active compound, toxic compound, or pollutant.
 72. A method according to claim 69, wherein the analyte is a polynucleotide.
 73. A method according to claim 72, wherein the polynucleotide comprises at least one homopolymeric region. 